python - be: Move exceptions to the Postgres.exceptions "module".

Log Message:
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Move exceptions to the Postgres.exceptions "module". Do so to avoid the clutter
of the many, many exceptions that maiden provides. This will, hopefully, be
reflected in the interface protocol at some point, as it provides a way to
easily identify the exceptions associated with a given interface module.

Move the error initialization portion to a new PyPgError_Initialize() function
in error.c for clarity. Also, improve Postgres.ERROR by allowing the 'code'
keyword. This was left off before as WARNING, NOTICE, etc dont take errcodes(?).
However, now, if the level < ERROR, just give it 0 regardless of the code it was
given..(hrm, maybe change this later..)
(Also, fix speeling in encoding.c)

Modified Files:
--------------
be/include/pypg:
error.h (r1.5 -> r1.6)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/include/pypg/error.h.diff?r1=1.5&r2=1.6)
be/src:
encoding.c (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/encoding.c.diff?r1=1.1&r2=1.2)
error.c (r1.8 -> r1.9)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/error.c.diff?r1=1.8&r2=1.9)
module.c (r1.18 -> r1.19)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/module.c.diff?r1=1.18&r2=1.19)
prime.py (r1.6 -> r1.7)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/prime.py.diff?r1=1.6&r2=1.7)