libpq now contains a mix of error message strings that end with newlines
and don't end with newlines, due to some newer code paths with new ways
of passing errors around. This has now gotten me confused a few too
many times both during development and translation. So I looked into
whether we can unify this, similar to how we have done elsewhere (e.g.,
pg_upgrade). I came up with the attached patch. It's not complete, but
it shows the idea and it looks like a nice simplification to me.
Thoughts on this approach?