Throw on invalid Conduit method parameters

Summary: NOTE: This may break stuff.

Test Plan:

$ echo '{}' | arc call-conduit conduit.ping
$ echo '{"x": 1}' | arc call-conduit conduit.ping

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4191