patch can be None in the exception, causing a secondary exception.

R=csharp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11232069

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@163646 0039d316-1c4b-4281-b951-d872f2087c98

checkout.py

@@ -76,7 +76,8 @@ class PatchApplicationFailed(Exception):
       out.append('Failed to apply patch for %s:' % self.filename)
     if self.status:
       out.append(self.status)
-    out.append('Patch: %s' % self.patch.dump())
+    if self.patch:
+      out.append('Patch: %s' % self.patch.dump())
     return '\n'.join(out)