Aframe: Add "request error" to materialtextureloaded, or error callback for texture loading

[Hulkmaster]

Description:
Right now if texture was not loaded with, for example, 404, callback is still fired, and there is no way to determine if texture was loaded or not

here texture loading error is handled
https://github.com/aframevr/aframe/blob/4a44a804bbf7347e41fdc88daa6fa1d68756029d/src/systems/material.js#L312

and herematerialtextureloaded event is fired
https://github.com/aframevr/aframe/blob/4a44a804bbf7347e41fdc88daa6fa1d68756029d/src/utils/material.js#L119

• A-Frame Version: 0.8.2

[Hulkmaster]

I think there might be several ways to make it appropriate:

1) (in my opinion) most efficient way: add error callback
2) don't fire loaded callback, if texture, in fact, was not loaded