Merge pull request #463 from matrix-org/daniel/hashtagnofilter

Skip, rather than erroring, invalid guest requests

Erroring causes problems when people make illegal requests, because they
don't know what limit parameter they should pass.

This is definitely buggy. It leaks message counts for rooms people don't
have permission to see, via tokens. But apparently we already
consciously decided to allow that as a team, so this preserves that
behaviour.