Improve error messages for CREATE OR REPLACE PROCEDURE

Improve error messages for CREATE OR REPLACE PROCEDURE

Change the hint to recommend DROP PROCEDURE instead of FUNCTION.  Also
make the error message when changing the return type more specific to
the case of procedures.

Reported-by: Jeremy Evans <code@jeremyevans.net>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>