Don't cause a traceback if cbfstool extraction failed. (Re: #398)

diffoscope/comparators/cbfs.py

@@ -87,7 +87,11 @@ class CbfsContainer(Archive):
             dest_path,
         ]
         logger.debug("cbfstool extract %s to %s", member_name, dest_path)
-        our_check_output(cmd, stderr=subprocess.DEVNULL)
+        try:
+            our_check_output(cmd, stderr=subprocess.DEVNULL)
+        except subprocess.CalledProcessError:
+            logger.error("unable to extract %s", member_name)
+            return None
         return dest_path