Call raise_for_status on hg.mozilla.org json-automationrelevance resp…

…onse

Makes more sense to get an http exception rather than a json decode error.

src/taskgraph/files_changed.py

@@ -49,6 +49,7 @@ def _get_changed_files_json_automationrelevance(head_repository_url, head_rev):
 
     def get_automationrelevance():
         response = requests.get(url, timeout=30)
+        response.raise_for_status()
         return response.json()
 
     contents = retry(get_automationrelevance, attempts=10, sleeptime=10)