python - pq: Replace asserts and keep determined fatality.

Log Message:
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Replace asserts and keep determined fatality.

Determine fatality to avoid redundancy where higher levels may need to
look at the 'fatal' attribute again and again.

Replace debugging assert with real, live exceptions: ParseError in element3, and
ProtocolError in client3.
[Also, fix bindings and executings' exception isolation again.]

Modified Files:
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pq/src:
client3.py (r1.19 -> r1.20)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/client3.py.diff?r1=1.19&r2=1.20)
element3.py (r1.13 -> r1.14)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/element3.py.diff?r1=1.13&r2=1.14)