Warn before parse errors in arc diff --verbatim

Summary: With `--verbatim` flag, notes created from parse errors are never displayed to user resulting in blank fields.

Test Plan:
- `arc diff --verbatim` with invalid Cc
- `arc diff --verbatim` with all fields correct

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2395

src/workflow/diff/ArcanistDiffWorkflow.php

@@ -1598,6 +1598,10 @@ EOTEXT
         $message->pullDataFromConduit($conduit, $partial = true);
         $fields += $message->getFields();
       } catch (ArcanistDifferentialCommitMessageParserException $ex) {
+        if ($this->getArgument('verbatim')) {
+          throw $ex;
+        }
+
         $fields += $message->getFields();
 
         $frev = substr($hash, 0, 8);