Bug 1572355 - Add a function to parse the .extra file r=ahal

Adds `parse_extra_file(path)` to simply read some json .extra file and return a json dict, or an empty dict if it raises a `ValueError`. In patches for Bug 1572244 this function is used.

Differential Revision: https://phabricator.services.mozilla.com/D59749

--HG--
extra : moz-landing-system : lando

testing/mozbase/mozcrash/mozcrash/mozcrash.py

@@ -5,6 +5,7 @@
 from __future__ import absolute_import, print_function
 
 import glob
+import json
 import os
 import re
 import shutil
@@ -349,6 +350,14 @@ class CrashInfo(object):
                          errors,
                          extra)
 
+    def _parse_extra_file(self, path):
+        with open(path) as file:
+            try:
+                return json.load(file)
+            except ValueError:
+                self.logger.warning(".extra file does not contain proper json")
+                return {}
+
     def _save_dump_file(self, path, extra):
         if os.path.isfile(self.dump_save_path):
             os.unlink(self.dump_save_path)