Error handling for missing Loom SDK / loom.bat

Otherwise there's an uncaught error each time you create a Loom project / try to build or run it when it can't find the Loom SDK / loom.bat.

I was wondering whether that error message needs to be localized. In `HaxeContext`'s `ValidateSDK()`, errors are handled the same way, though.

Btw, shouldn't the Appmanager have the Loom SDK?