python - lo: Remove redundancy in Error's __str__ method.

Log Message:
-----------
Remove redundancy in Error's __str__ method.

Don't show message and severity as severity is at least, and normally, ERROR,
and message is already shown.
Provide a more concise printing of the location when 'file', 'line', or
'function' is available.

Modified Files:
--------------
lo/postgresql:
exceptions.py (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/lo/postgresql/exceptions.py.diff?r1=1.1&r2=1.2)