Bug 1467215 - Abort initialisation on fatal error. r=maja_zf

When the Marionette component fails to initialise we fail to bail
out.  This causes Marionette to report itself as enabled through
an environment variable, an observer notification, and a log message.

MozReview-Commit-ID: 2nTNNP0o5dv

--HG--
extra : rebase_source : 3b780bf2f24e03ba796d50cf5094c28d951847ec

testing/marionette/components/marionette.js

@@ -449,6 +449,7 @@ class MarionetteMainProcess {
         log.fatal("Remote protocol server failed to start", e);
         this.uninit();
         Services.startup.quit(Ci.nsIAppStartup.eForceQuit);
+        return;
       }
 
       env.set(ENV_ENABLED, "1");