revsetlang: add a hint for more useful parse errors

Authored by ryanmce.

Description

revsetlang: add a hint for more useful parse errors

This logic is largely based on the similar logic added to template error
messages in D2608 and D2609, but with a few tweaks based on how revsets
actually work.

Differential Revision: https://phab.mercurial-scm.org/D2619

Details