Add new foreign key constraint error codes.

See https://github.com/doctrine/dbal/commit/5df3a64213b2a57504826324dc8fbdd0cb6655bf
Locked new DBAL commit.
Added new test DBAL\Functional\ConnectionTest.

Informix doesn't support a connection without a explicit database name so
the platform is excluded from the test testConnectWithoutExplicitDatabaseName().