Fix bug:

[ 1327110 ] wrong TypeError traceback in generator expressions

by removing the code that can stomp on the users' TypeError raised by the
iterable argument to ''.join() -- PySequence_Fast (now?) gives a perfectly
reasonable message itself.  Also, a couple of tests.
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 9f092b9..60f5fdb 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -657,6 +657,15 @@
         self.checkraises(TypeError, ' ', 'join')
         self.checkraises(TypeError, ' ', 'join', 7)
         self.checkraises(TypeError, ' ', 'join', Sequence([7, 'hello', 123L]))
+        try:
+            def f():
+                yield 4 + ""
+            self.fixtype(' ').join(f())
+        except TypeError, e:
+            if '+' not in str(e):
+                self.fail('join() ate exception message')
+        else:
+            self.fail('exception not raised')
 
     def test_formatting(self):
         self.checkequal('+hello+', '+%s+', '__mod__', 'hello')