Fix bogus handling of bad strategy number in GIST consistent() functions.

Fix bogus handling of bad strategy number in GIST consistent() functions.

Make sure we throw an error instead of silently doing the wrong thing when
fed a strategy number we don't recognize.  Also, in the places that did
already throw an error, spell the error message in a way more consistent
with our message style guidelines.

Per report from Paul Jones.  Although this is a bug, it won't occur unless
a superuser tries to do something he shouldn't, so it doesn't seem worth
back-patching.