faft: Show the RPC address in some socket errors.

Before, it could be unclear what connection was refused:
[Errno 111] Connection refused

Now, the port number suggests that it's a local servod:
[Errno 111] Connection refused: 'localhost:9999'

BUG=None
TEST=Run firmware_FAFTSetup with local servod not running

Change-Id: I840fc792142662ce0cd82def3d0b506ddc936dc7
Reviewed-on: https://chromium-review.googlesource.com/1682167
Tested-by: Dana Goyette <dgoyette@chromium.org>
Commit-Ready: Dana Goyette <dgoyette@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Greg Edelston <gredelston@google.com>
diff --git a/server/cros/servo/servo.py b/server/cros/servo/servo.py
index 45df8be..7e2d017 100644
--- a/server/cros/servo/servo.py
+++ b/server/cros/servo/servo.py
@@ -9,6 +9,7 @@
 import logging
 import os
 import re
+import socket
 import time
 import xmlrpclib
 
@@ -260,6 +261,7 @@
 
         @param servo_host: A ServoHost object representing
                            the host running servod.
+        @type servo_host: autotest_lib.server.hosts.servo_host.ServoHost
         @param servo_serial: Serial number of the servo board.
         """
         # TODO(fdeng): crbug.com/298379
@@ -310,7 +312,12 @@
         @param cold_reset If True, cold reset the device after
                           initialization.
         """
-        self._server.hwinit()
+        try:
+            self._server.hwinit()
+        except socket.error as e:
+            e.filename = '%s:%s' % (self._servo_host.hostname,
+                                    self._servo_host.servo_port)
+            raise
         self.set('usb_mux_oe1', 'on')
         self._usb_state = None
         self.switch_usbkey('off')