Improve enlargeStringInfo's ERROR message

Improve enlargeStringInfo's ERROR message

Until now, when an enlargeStringInfo() call would cause the StringInfo to
exceed its maximum size, we reported an "out of memory" error.  This is
misleading as it's no such thing.

Here we remove the "out of memory" text and replace it with something
more relevant to better indicate that it's a program limitation that's
been reached.

Reported-by: Michael Banck
Reviewed-by: Daniel Gustafsson, Tom Lane
Discussion: https://postgr.es/m/18484-3e357ade5fe50e61@postgresql.org