buildman: show base error for buildpack failures

Whereas before these were reserved only for S3 errors, users need these
specifics to debug custom-git configurations.

buildman/jobutil/workererror.py

@@ -7,7 +7,8 @@ class WorkerError(object):
     self._error_handlers = {
         'io.quay.builder.buildpackissue': {
             'message': 'Could not load build package',
-            'is_internal': True
+            'is_internal': True,
+            'show_base_error': True,
         },
 
         'io.quay.builder.cannotextractbuildpack': {