Report 404 errors from Gerrit and continue.

Currently, we get a 404 from Gerrit in the event that two projects
have the same Change-Id. We should be able to handle this and actually
cherry-pick each change so we can check multi-project changes, but for
now just skip these changes.

Change-Id: I7bc63208998d58beec83b71b302450d9be3ea026

tools/bionicbb/gmail_listener.py

@@ -297,6 +297,13 @@ def process_message(msg, dry_run):
     except NotImplementedError as ex:
         print ex
         return False
+    except gerrit.GerritError as ex:
+        if ex.code == 404:
+            print '{}(404): {}!'.format(
+                termcolor.colored('ERROR', 'red'), ex)
+            return True
+        else:
+            return False
 
 
 def main(argc, argv):