Show more-intuitive titles for psql commands \dt, \di, etc.

Show more-intuitive titles for psql commands \dt, \di, etc.

If exactly one relation type is requested in a command of the \dtisv
family, say "tables", "indexes", etc instead of "relations".  This
should cover the majority of actual uses, without creating a huge
number of new translatable strings.  The error messages for no
matching relations are adjusted as well.

In passing, invent "pg_log_error_internal()" to be used for frontend
error messages that don't seem to need translation, analogously to
errmsg_internal() in the backend.  The implementation is a bit cheesy,
being just a macro to prevent xgettext from recognizing a trigger
keyword.  This won't avoid a useless gettext lookup cycle at runtime
--- but surely we don't care about an extra microsecond or two in
what's supposed to be a can't-happen case.  I (tgl) also made
"pg_fatal_internal()", though it's not used in this patch.

Author: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAKAnmm+7o93fQV-RFkGaN1QnP-0D4d3JTykD+cLueqjDMKdfag@mail.gmail.com