Issue #618 - Align error handling for module scripts with the spec (again)

This updates module implementation to match spec regarding handling of
instantiation errors, after it was changed yet again, this time to not remember
instantiation errors, but instead immediately rethrow applicable ones.

Ref: BZ 1420420

dom/script/ModuleLoadRequest.cpp

@@ -83,7 +83,7 @@ ModuleLoadRequest::ModuleLoaded()
   // been loaded.
 
   mModuleScript = mLoader->GetFetchedModule(mURI);
-  if (!mModuleScript || mModuleScript->IsErrored()) {
+  if (!mModuleScript || mModuleScript->HasParseError()) {
     ModuleErrored();
     return;
   }
@@ -95,7 +95,7 @@ void
 ModuleLoadRequest::ModuleErrored()
 {
   mLoader->CheckModuleDependenciesLoaded(this);
-  MOZ_ASSERT(!mModuleScript || mModuleScript->IsErrored());
+  MOZ_ASSERT(!mModuleScript || mModuleScript->HasParseError());
 
   CancelImports();
   SetReady();