Fix logical replication slot initialization

This was broken in commit 9c7d06d6, which inadvertently gave the
wrong value to fast_forward in one StartupDecodingContext call.  Fix by
flipping the value.  Add a test for the obvious error, namely trying to
initialize a replication slot with an nonexistent output plugin.

While at it, move the CreateDecodingContext call earlier, so that any
errors are reported before sending the CopyBoth message.

Author: Dave Cramer <davecramer@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CADK3HHLVkeRe1v4P02-5hj55H3_yJg3AEtpXyEY5T3wuzO2jSg@mail.gmail.com