python - pq: Fix error handling in Query and Function.

Log Message:
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Fix error handling in Query and Function.

The transactions, specifically Query, were propagating the state to a
non-existing method. The problem here is that that raised a Transaction
exception that made the interface(proboscis) think that it was an unhandled
exception/error. That was a problem as Query and Function were meant to "handle"
their error message and following autosync Ready message.

Also, make some protocol exceptions more descriptive.

Modified Files:
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    pq/src:
        client3.py (r1.15 -> r1.16)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/client3.py.diff?r1=1.15&r2=1.16)