Throw a more tailored exception after failing to resolve a ref

Summary: Ref T2683. Throw a more tailored exception to allow callers to distinguish between bad refs (which are expected, if users try to visit garbage branches) and other types of errors.

Test Plan: Tried to view branch "alksndfklansdf". Viewed branch "master".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9094

src/applications/diffusion/exception/DiffusionRefNotFoundException.php

@@ -0,0 +1,16 @@
+<?php
+
+final class DiffusionRefNotFoundException extends Exception {
+
+  private $ref;
+
+  public function setRef($ref) {
+    $this->ref = $ref;
+    return $this;
+  }
+
+  public function getRef() {
+    return $this->ref;
+  }
+
+}