R1:36d02b6c798b

Fix a use of $method in Conduit which is not available in scope

Summary:
This got refactored at some point and lost access to $method. Also make the error a little more helpful.

See https://groups.google.com/forum/?fromgroups=#!topic/phabricator-dev/05voYIPV7uU

Test Plan:
$ arc list --conduit-uri=http://local.aphront.com:8080/
Exception
ERR-CONDUIT-CORE: Invalid parameter information was passed to method 'conduit.connect', could not decode JSON serialization. Data: xxx{"client":"arc"…