Skipping useless ereport/elog calls.

Skipping useless ereport/elog calls.

Although ereport() and elog() themselves are quite cheap when the error message
level is too low to be printed, some places need to do substantial work before
they can call those macros at all. message_level_is_interesting() can be handy
to allow optimizing away such setup work when nothing is to be printed.

Function message_level_is_interesting(elevel) is borrowed from PostgreSQL
source that reports whether ereport/elog will do anything.