Add missing error handling in pg_md5_hash().

Add missing error handling in pg_md5_hash().

It failed to provide an error string as expected for the
admittedly-unlikely case of OOM in pg_cryptohash_create().
Also, make it initialize *errstr to NULL for success,
as pg_md5_binary() does.

Also add missing comments.  Readers should not have to
reverse-engineer the API spec for a publicly visible routine.