Gracefully fail request if non existing callsign is passed to getrecentcommitsbypath instead of crashing

Summary:
`diffusion.getrecentcommitsbypath` fails with 500 error when non existing callsign is passed:
```
>>> UNRECOVERABLE FATAL ERROR <<<

Call to a member function getCommit() on null

```

Expected Behavior:
Return more graceful error notifying caller that such callsign/repository does not exist

Reproduction steps:
Open conduit: https://secure.phabricator.com/conduit/method/diffusion.getrecentcommitsbypath/
Enter:
callsign: "obviouslynotexisting"
path: "/random"
Click call method

Test Plan: after applying patch - call no longer fails with 500s

Reviewers: Pawka, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19558

src/applications/diffusion/conduit/DiffusionGetRecentCommitsByPathConduitAPIMethod.php

@@ -23,6 +23,12 @@ final class DiffusionGetRecentCommitsByPathConduitAPIMethod
     );
   }
 
+  protected function defineErrorTypes() {
+    return array(
+      'ERR_NOT_FOUND' => pht('Repository was not found.'),
+    );
+  }
+
   protected function defineReturnType() {
     return 'nonempty list<string>';
   }
@@ -36,6 +42,10 @@ final class DiffusionGetRecentCommitsByPathConduitAPIMethod
         'branch' => $request->getValue('branch'),
       ));
 
+    if ($drequest === null) {
+        throw new ConduitException('ERR_NOT_FOUND');
+    }
+
     $limit = nonempty(
       $request->getValue('limit'),
       self::DEFAULT_LIMIT);