Translate psycopg2 errors/exceptions into tooz exceptions

Instead of raising raw psycopg2 errors/exceptions from the
postgresql driver we should make an attempt to translate
those errors/exceptions into a equivalent tooz exception to
make it so that users of tooz can handle the known driver
failure modes in a sane manner (users expecting to user tooz
as a pluggable abstraction will not be able to handle raw
psycopg2 errors/exceptions, especially if those users are
using different tooz backends at the same time).

Change-Id: Ic95e5fa21bb25096a31b5c5e38aa0b14afa05d87
Merge "Translate psycopg2 errors/exceptions into tooz exceptions"