Handle and return non-success status codes when loading files on web

[lunabunn]

Successor of #284

• Add WebAssetLoadingError(u16) to fs::Error a la AndroidAssetLoadingError
• Call file loaded callback even on failure (fixes infinite hang when file fails to load)
• Throw fs::Error::DownloadFailed on 404 (file not found), otherwise throw fs::Error::WebAssetLoadingError with the status code
• Treat XMLHttpRequest errors as a 404 (file not found) and also throw fs::Error::DownloadFailed

Everything is tested, but I'm leaving this as a draft because:

• This is a breaking change; should the gl.js version be bumped to 0.1.27?
• This may return garbage if the server responds with an invalid status code (such as negative status codes or status codes over i16::MAX); I think this is fine, but do you want me to special-case this and throw DownloadFailed?

[lunabunn]

@not-fl3 I already asked on Discord a few months ago, but can I please get your input on this? I have un-drafted it in case that's what was stopping you.

I see that on master you have a different solution for this, but the JS code there does not gracefully handle non-success error codes and is not very idiomatic overall. I think this is a nicer approach.

As stated in the PR itself, this is mostly ready to merge. Just waiting for review.

[not-fl3]

Thanks for pr! Will look closely into this after 0.4 merge!