More helpful error if exemplar stream doesn't exist

nilmtools/trainola.py

@@ -54,6 +54,9 @@ class Exemplar(object):
         # Get stream info
         self.client = nilmdb.client.numpyclient.NumpyClient(self.url)
         self.info = nilmtools.filter.get_stream_info(self.client, self.stream)
+        if not self.info:
+            raise DataError(sprintf("exemplar stream '%s' does not exist " +
+                                    "on server '%s'", self.stream, self.url))
 
         # Build up name => index mapping for the columns
         self.columns = build_column_mapping(exinfo['columns'], self.info)