When using pg_dump --strict-names and a schema pattern which doesn't
match any schemas (eg: --schema='nonexistant*'), we were incorrectly
throwing an error claiming no tables were found when, really, there
were no schemas found:

  -> pg_dump --strict-names --schema='nonexistant*'
  pg_dump: no matching tables were found for pattern "nonexistant*"

Fix that by changing the error message to say 'schemas' instead, since
that is what we are actually complaining about.

Noticed while testing pg_dump error cases.

Back-patch to 9.6 where --strict-names and this error message were
introduced.