python - be: Add catch and end convenience macro.

Log Message:
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Add catch and end convenience macro.

This will get use in various places where all that is left to do after the
PyErr_SetPostgresError() is a return(NULL) or a goto some_cleanup.

Modified Files:
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be/include/pypg:
error.h (r1.2 -> r1.3)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/include/pypg/error.h.diff?r1=1.2&r2=1.3)