Bug 723561 - TelemetryStopwatch follow-up: display key name on error. r=gavin

toolkit/components/telemetry/TelemetryStopwatch.jsm

@@ -49,7 +49,8 @@ let TelemetryStopwatch = {
 
     if (timers.hasOwnProperty(aHistogram)) {
       delete timers[aHistogram];
-      Cu.reportError("TelemetryStopwatch: key was already initialized");
+      Cu.reportError("TelemetryStopwatch: key \"" +
+                     aHistogram + "\" was already initialized");
       return false;
     }