Improve imap error message in unusual failure mode.

We ran into this during the sprits at PyCon and this patch has been
sitting on my disk ever since.  This just adds some information to the
error message that we found useful during debugging.  There's no good
way to add a test, since the message only got generated via code
that we had modified for debugging purposes.

Lib/imaplib.py

@@ -1073,7 +1073,7 @@ class IMAP4:
 
         # Protocol mandates all lines terminated by CRLF
         if not line.endswith(b'\r\n'):
-            raise self.abort('socket error: unterminated line')
+            raise self.abort('socket error: unterminated line: %r' % line)
 
         line = line[:-2]
         if __debug__: