Make exception reporting from arc be in red

Summary:
Many other status updates (such as "Builds passed!") show up
in bright background colors, making them more salient than a final
fatal "Exception".

Make exception reporting be just as colorful, so it stands out.

Test Plan:
Added an explicit `throw new Exception("!!!")` and saw it
in red.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17748

scripts/arcanist.php

@@ -422,7 +422,8 @@ try {
   }
 
   if (!$is_usage) {
-    fwrite(STDERR, phutil_console_format("**%s**\n", pht('Exception')));
+    fwrite(STDERR, phutil_console_format(
+      "<bg:red>** %s **</bg>\n", pht('Exception')));
 
     while ($ex) {
       fwrite(STDERR, $ex->getMessage()."\n");