Invalid json in case of failed load

https://github.com/playcanvas/engine/blob/18bdc36d108031beca3fbfab948feb911eea2159/src/net/http.js#L372

If I try to load a JSON file and I have a loading error (for example "No Access-Control-Allow-Origin header is present") the xhr status is 0 and is considered as a success. I don't know if this is correct, but supposing it is, the result of the get request is an empty string that cannot be parsed:

Uncaught SyntaxError: Unexpected end of JSON input
at JSON.parse ()
at Object._onSuccess (playcanvas-stable.dbg.js:20204)
at Object._onReadyStateChange (playcanvas-stable.dbg.js:20174)
at Object. (playcanvas-stable.dbg.js:20138)

So a simple patch would be to check for a not empty string before parsing, but I'm not sure if it's not better to understand why the xhr status zero is considered a successful request.
I had also an issue when the network was off. The get method doesn't return an error, but a successful empty result.

Start in src/net/http.js at line 372 and reproduce a JSON load with a CORS failure or the network offline. Trace how an XHR status of 0 and an empty response are handled before JSON.parse. Done means failed loads report an error instead of being treated as successful empty JSON results.