Don't tolerate BindComplete after Parse + Describe + Sync

There's no real world scenario where this would happen.
Don't tolerate CommandComplete after Parse + Describe + Sync

Commit 13601feaf94cbd2df4a9b0b752150e08c5885f72 introduced this change,
(after being reported in issue #77), but the problem does not seem to
be with pq misbehaving; the test case itself is to blame.  It does not
correctly close the Rows object before attempting to operate on the
transaction again, which the "Extended query" mode does not like in
the slightest.

The TestRollback test also only tested the simpleQuery path, which
did not have a problem to begin with because it eats the
CommandComplete message from the server if the query returns no
results.  Get rid of the defective test case, and add a new one
testing an error response to the Parse message.