Warn for file processing failures

[samme]

I reproduced #5851. If you load an aseprite animation from an invalid image (e.g. wrong type or zero-length file), the files are complete, the animations are missing, but there are no other errors.

Generally I think the problem is that most file process failures are silent, which is a little confusing for users.

https://codepen.io/samme/pen/yLoLJVE?editors=0010

[photonstorm]

Thanks for opening this issue, and for submitting a PR to fix it. We have merged your PR into the master branch and attributed the work to you in the Change Log. If you need to tweak the code for whatever reason please submit a new PR.