[dataformat] Make export error message more verbose

beat/backend/python/dataformat.py

@@ -515,7 +515,7 @@ class DataFormat(object):
             raise RuntimeError("dataformat has no name")
 
         if not self.valid:
-            raise RuntimeError("dataformat is not valid")
+            raise RuntimeError("dataformat is not valid:\n{}".format(self.errors))
 
         if prefix == self.prefix:
             raise RuntimeError(