R1:a0e93fd4c44c

Provide a separate error code for invalid Conduit calls

Summary: I plan to use this in Arcanist.

Test Plan:
$ echo '{}' | arc call-conduit x

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4192