Improve error report for PL/pgSQL reserved word used as a field name.

Improve error report for PL/pgSQL reserved word used as a field name.

The current code in resolve_column_ref (dating to commits 01f7d2990
and fe24d7816) believes that not finding a RECFIELD datum is a
can't-happen case, in consequence of which I didn't spend a whole lot
of time considering what to do if it did happen.  But it turns out
that it *can* happen if the would-be field name is a fully-reserved
PL/pgSQL keyword.  Change the error message to describe that
situation, and add a test case demonstrating it.

This might need further refinement if anyone can find other ways to
trigger a failure here; but without an example it's not clear what
other error to throw.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://postgr.es/m/2185258.1745617445@sss.pgh.pa.us