ensure wptrunner.py start() always returns a value

Differential Revision: https://phabricator.services.mozilla.com/D52191

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1594069
gecko-commit: 0b0779e6ebe39537f30b6bed33a0201f0434d308
gecko-integration-branch: autoland
gecko-reviewers: jgraham
diff --git a/tools/wptrunner/wptrunner/wptrunner.py b/tools/wptrunner/wptrunner/wptrunner.py
index 7dfd86e..cac172a 100644
--- a/tools/wptrunner/wptrunner/wptrunner.py
+++ b/tools/wptrunner/wptrunner/wptrunner.py
@@ -342,6 +342,7 @@
     handler = handlers.LogLevelFilter(logged_critical, "CRITICAL")
     logger.add_handler(handler)
 
+    rv = False
     try:
         if kwargs["list_test_groups"]:
             list_test_groups(**kwargs)
@@ -350,11 +351,12 @@
         elif kwargs["list_tests"]:
             list_tests(**kwargs)
         elif kwargs["verify"] or kwargs["stability"]:
-            return check_stability(**kwargs) or logged_critical.has_log
+            rv = check_stability(**kwargs) or logged_critical.has_log
         else:
-            return not run_tests(**kwargs) or logged_critical.has_log
+            rv = not run_tests(**kwargs) or logged_critical.has_log
     finally:
         logger.remove_handler(handler)
+    return rv
 
 
 def main():