serial.c: Show both error code and error message.

serial.c: Show both error code and error message.

This is helpful in many cases, e.g. when trying to identify which of the
16000 system error codes from

  http://msdn.microsoft.com/en-us/library/ms681381%28VS.85%29.aspx

has been encountered (which is not trivial if you only have an,
e.g. German, string message alone).