Fail fast in `pw.io.postgres.read` if the publication doesn't exist

[zxqfd555]

Is your feature request related to a problem? Please describe.
The Postgres input connector requires a publication_name for streaming mode. Currently, it is possible to pass the name of a nonexistent publication. The execution will fail, but without a clear or concise error message.

Describe the solution you'd like
This should be fixed: the check must be performed in PsqlReader::new, and the resulting error must be human-readable.