Bug 1918449 - Part 1: Handle unknown module type earlier r=jonco,dom-worker-reviewers,dom-core,asuth,mccr8

We never reach `CompileFetchedModule` if we have an unknown module type since
we take the module type into account when we do mime type checking, so the file
always fails to load because of MIME mismatch, and we don't get the expected
`TypeError.` Checking if we have an unknown type should happen much earlier in
the process.

Differential Revision: https://phabricator.services.mozilla.com/D222222

dom/script/ModuleLoader.cpp

@@ -204,9 +204,7 @@ nsresult ModuleLoader::CompileFetchedModule(
 
   switch (aRequest->mModuleType) {
     case JS::ModuleType::Unknown:
-      JS_ReportErrorNumberASCII(aCx, js::GetErrorMessage, nullptr,
-                                JSMSG_BAD_MODULE_TYPE);
-      return NS_ERROR_FAILURE;
+      MOZ_CRASH("Unexpected module type");
     case JS::ModuleType::JavaScript:
       return CompileJavaScriptModule(aCx, aOptions, aRequest, aModuleOut);
     case JS::ModuleType::JSON: {