Use the errorsummary data source as a fallback for the Treeherder data source

Up to mozci 2.0.0, the fallback was not in a separate data source

infra/mozci_config.toml

@@ -1,5 +1,5 @@
 [mozci]
-data_sources = ["treeherder_client", "hgmo", "taskcluster"]
+data_sources = ["treeherder_client", "hgmo", "taskcluster", "errorsummary"]
 
 [mozci.cache]
 retention = 40320