refactor: raise new instances for PortNotOpenError and SerialTimeoutException

related to #502
fixes #437
diff --git a/serial/serialposix.py b/serial/serialposix.py
index 334ba9f..f31b99d 100644
--- a/serial/serialposix.py
+++ b/serial/serialposix.py
@@ -39,7 +39,7 @@
 
 import serial
 from serial.serialutil import SerialBase, SerialException, to_bytes, \
-    portNotOpenError, writeTimeoutError, Timeout
+    PortNotOpenError, SerialTimeoutException, Timeout
 
 
 class PlatformSpecificBase(object):
@@ -532,7 +532,7 @@
         until the requested number of bytes is read.
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         read = bytearray()
         timeout = Timeout(self._timeout)
         while len(read) < size:
@@ -585,7 +585,7 @@
     def write(self, data):
         """Output the given byte string over the serial port."""
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         d = to_bytes(data)
         tx_len = length = len(d)
         timeout = Timeout(self._write_timeout)
@@ -600,13 +600,13 @@
                     # when timeout is set, use select to wait for being ready
                     # with the time left as timeout
                     if timeout.expired():
-                        raise writeTimeoutError
+                        raise SerialTimeoutException('Write timeout')
                     abort, ready, _ = select.select([self.pipe_abort_write_r], [self.fd], [], timeout.time_left())
                     if abort:
                         os.read(self.pipe_abort_write_r, 1000)
                         break
                     if not ready:
-                        raise writeTimeoutError
+                        raise SerialTimeoutException('Write timeout')
                 else:
                     assert timeout.time_left() is None
                     # wait for write operation
@@ -633,7 +633,7 @@
                 if e[0] not in (errno.EAGAIN, errno.EALREADY, errno.EWOULDBLOCK, errno.EINPROGRESS, errno.EINTR):
                     raise SerialException('write failed: {}'.format(e))
             if not timeout.is_non_blocking and timeout.expired():
-                raise writeTimeoutError
+                raise SerialTimeoutException('Write timeout')
         return length - len(d)
 
     def flush(self):
@@ -642,13 +642,13 @@
         is written.
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         termios.tcdrain(self.fd)
 
     def reset_input_buffer(self):
         """Clear input buffer, discarding all that is in the buffer."""
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         termios.tcflush(self.fd, termios.TCIFLUSH)
 
     def reset_output_buffer(self):
@@ -657,7 +657,7 @@
         that is in the buffer.
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         termios.tcflush(self.fd, termios.TCOFLUSH)
 
     def send_break(self, duration=0.25):
@@ -666,7 +666,7 @@
         duration.
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         termios.tcsendbreak(self.fd, int(duration / 0.25))
 
     def _update_rts_state(self):
@@ -687,7 +687,7 @@
     def cts(self):
         """Read terminal status line: Clear To Send"""
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
         return struct.unpack('I', s)[0] & TIOCM_CTS != 0
 
@@ -695,7 +695,7 @@
     def dsr(self):
         """Read terminal status line: Data Set Ready"""
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
         return struct.unpack('I', s)[0] & TIOCM_DSR != 0
 
@@ -703,7 +703,7 @@
     def ri(self):
         """Read terminal status line: Ring Indicator"""
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
         return struct.unpack('I', s)[0] & TIOCM_RI != 0
 
@@ -711,7 +711,7 @@
     def cd(self):
         """Read terminal status line: Carrier Detect"""
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
         return struct.unpack('I', s)[0] & TIOCM_CD != 0
 
@@ -730,7 +730,7 @@
         WARNING: this function is not portable to different platforms!
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         return self.fd
 
     def set_input_flow_control(self, enable=True):
@@ -740,7 +740,7 @@
         WARNING: this function is not portable to different platforms!
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         if enable:
             termios.tcflow(self.fd, termios.TCION)
         else:
@@ -753,7 +753,7 @@
         WARNING: this function is not portable to different platforms!
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         if enable:
             termios.tcflow(self.fd, termios.TCOON)
         else:
@@ -779,7 +779,7 @@
         until the requested number of bytes is read.
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         read = bytearray()
         timeout = Timeout(self._timeout)
         poll = select.poll()
@@ -856,7 +856,7 @@
         until the requested number of bytes is read.
         """
         if not self.is_open:
-            raise portNotOpenError
+            raise PortNotOpenError()
         read = bytearray()
         while len(read) < size:
             buf = os.read(self.fd, size - len(read))