Bug 1539449 - [mozharness] Ensure content crashes turn task orange even if no stack was printed, r=jmaher

There's a failure case where content processes are crashing but no stack trace
is being printed. Test harnesses often rely on the presence of a minidump to
determine whether or not there was a crash, and so in this failure mode report
success (so tasks are staying green).

This adds a new string to mozharness' error logs to make sure the task turns
orange. Note: it does not fix the lack of stack traces.

Differential Revision: https://phabricator.services.mozilla.com/D31635

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

testing/mozharness/mozharness/mozilla/testing/errors.py

@@ -124,6 +124,7 @@ TestPassed = [
 HarnessErrorList = [
     {'substr': 'TEST-UNEXPECTED', 'level': ERROR, },
     {'substr': 'PROCESS-CRASH', 'level': ERROR, },
+    {'substr': 'A content process crashed', 'level': ERROR, },
     {'regex': re.compile('''ERROR: (Address|Leak)Sanitizer'''), 'level': ERROR, },
     {'regex': re.compile('''thread '([^']+)' panicked'''), 'level': ERROR, },
     {'substr': 'pure virtual method called', 'level': ERROR, },