Update error for plugin when querying against DB that doesn't exist

When running `query_rows` against a db that doesn't exist, it will give a strange validation error. Instead should return error that the DB is missing.

Start by reproducing the issue with `query_rows` against a database that does not exist, then trace the validation error to the relevant query entry point. The change is complete when the missing database produces a clear database-missing error instead of the current validation error; the payload names no files or tests to run.

Written by the indexing model from the issue text.