bootstrap: usable error on missing internal module

Due to how bootstrap/loaders.js itself is loaded and invoked,
stacktraces from it are munged and no longer point back to the error
source.

That resulted in the following unhelpful error if an internal module
was missing or misnamed:

```
internal/bootstrap/loaders.js:190
  return mod.compile();
             ^

TypeError: Cannot read property 'compile' of undefined
```

This changes that to at least print the id that was attempted to be
loaded:

```
internal/bootstrap/loaders.js:189
  if (!mod) throw new TypeError(`Missing internal module '${id}'`);
            ^

TypeError: Missing internal module 'internal/a'
```

Author: Fishrock123

lib/internal/bootstrap/loaders.js

@@ -186,6 +186,9 @@ function nativeModuleRequire(id) {
   }
 
   const mod = NativeModule.map.get(id);
+  // Can't load the internal errors module from here, have to use a raw error.
+  // eslint-disable-next-line no-restricted-syntax
+  if (!mod) throw new TypeError(`Missing internal module '${id}'`);
   return mod.compile();
 }