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

src/applications/conduit/call/ConduitCall.php

@@ -70,13 +70,13 @@ final class ConduitCall {
       // Discard, we provide a more specific exception below.
     }
     if (!$ok) {
-      throw new Exception(
+      throw new ConduitException(
         "Conduit method '{$method}' does not exist.");
     }
 
     $class_info = new ReflectionClass($method_class);
     if ($class_info->isAbstract()) {
-      throw new Exception(
+      throw new ConduitException(
         "Method '{$method}' is not valid; the implementation is an abstract ".
         "base class.");
     }

src/applications/conduit/controller/PhabricatorConduitAPIController.php

@@ -108,7 +108,9 @@ final class PhabricatorConduitAPIController
     } catch (Exception $ex) {
       phlog($ex);
       $result = null;
-      $error_code = 'ERR-CONDUIT-CORE';
+      $error_code = ($ex instanceof ConduitException
+        ? 'ERR-CONDUIT-CALL'
+        : 'ERR-CONDUIT-CORE');
       $error_info = $ex->getMessage();
     }