Note that cPickleCache.c must be recompiled else the new test will fail.

Change the exception raised when an attempt is made to add two objects to
the cache with the same oid.  The former messsage didn't make sense.

Add a test to verify that this exception does get raised, and that the
message given is the intended one.

This is the first of a series of checkins, to fix critical bugs where
ZODB can in fact raise this exception in rare, but normal, use cases.