Bug 1277104 - Add strings to high-frequency MOZ_CRASH() occurrences. mccr8.

Some of these are guesses; it's not always clear from a crash report stack
trace which MOZ_CRASH() was hit.

xpcom/base/CycleCollectedJSRuntime.cpp

@@ -505,7 +505,7 @@ CycleCollectedJSRuntime::Initialize(JSRuntime* aParentRuntime,
   }
 
   if (!JS_AddExtraGCRootsTracer(mJSRuntime, TraceBlackJS, this)) {
-    MOZ_CRASH();
+    MOZ_CRASH("JS_AddExtraGCRootsTracer failed");
   }
   JS_SetGrayGCRootsTracer(mJSRuntime, TraceGrayJS, this);
   JS_SetGCCallback(mJSRuntime, GCCallback, this);