Require Conduit methods to be instances of ConduitAPIMethod

Test Plan:
Deleted `extends ConduitAPIMethod`, then:

  $ echo '{}' | arc call-conduit conduit.ping

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4190

src/applications/conduit/call/ConduitCall.php

@@ -81,7 +81,15 @@ final class ConduitCall {
         "base class.");
     }
 
-    return newv($method_class, array());
+    $method = newv($method_class, array());
+
+    if (!($method instanceof ConduitAPIMethod)) {
+      throw new ConduitException(
+        "Method '{$method}' is not valid; the implementation must be ".
+        "a subclass of ConduitAPIMethod.");
+    }
+
+    return $method;
   }