Improve stability of subscription/029_on_error.pl

Improve stability of subscription/029_on_error.pl

This test was failing when using wal_debug=on and -DWAL_DEBUG because of
additional log entries that made the test grab an LSN not mapping with
the error expected in the test.

Previously the test would look for the first matching line to get the
LSN to skip up to.  This is improved by having the test scan the logs
with a regexp that checks for the expected ERROR string, ensuring that
the wanted LSN comes from the correct context.

Backpatch down to 15 where this test has been introduced.

Author: Ian Ilyasov
Discussion: https://postgr.es/m/GV1P251MB100415F17E6B2FDD7188777ECDE32@GV1P251MB1004.EURP251.PROD.OUTLOOK.COM
Backpatch-through: 15