python - ex: Use 'code' instead of 'ec' to enhance clarity.

Log Message:
-----------
Use 'code' instead of 'ec' to enhance clarity.
(Also for coming checkin, to allow Postgres.exceptions.code.X)

Modified Files:
--------------
ex/src:
prime.py (r1.4 -> r1.5)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/ex/src/prime.py.diff?r1=1.4&r2=1.5)