buildman: add git checkout failure

buildman/jobutil/workererror.py

@@ -15,6 +15,10 @@ class WorkerError(object):
             'show_base_error': True,
         },
 
+        'io.quay.builder.gitcheckout': {
+            'message': 'Could not checkout git ref. Have you force pushed recently?',
+        },
+
         'io.quay.builder.cannotextractbuildpack': {
             'message': 'Could not extract the contents of the build package'
         },