Remove spurious semicolon from the end of default ExtraSysTablePrefixes.

The code that forms a new connection string from current settings doesn't
escape it correctly, so that the connection string looks like
"...;C2=dd_;;CX=...". Of course, the code that forms the connection string
should be fixed, but that's a much bigger job.
Merge remote-tracking branch 'origin/master' into libpq-integration4
Add a test case for error handling.

The current behavior isn't actually quite correct with some configurations,
but seems better to add the regression test now anyway, even though the
errors_2.out basically memorizes the bogus behavior. It should be removed
when the bug is fixed.
Attempt to fix behavior if an error is received from the backend.

Don't throw away the error message...
Merge branch 'master' into libpq-integration5