Display correct error on max-files-watch

[0xpr03]

This is a draft to display the correct error message when maximum number of files watchable is reached. Under linux you can test this by using the default example and setting sudo sysctl -w fs.inotify.max_user_watches=1

It currently works as expected on

• linux
• Windows
• MacOS

Windows does not seem to expose some kinds of limits on its API.

I don't have access to a mac system but they have limit, so it'd be nice to know how its working over there (possibly the same due to libc?). Windows will probably behave completely different. See down below for why macos doesn't seem to need any changes.

Fixes #266

[0xpr03]

On a separate note: We might want to have a list of people than can test stuff like glutin/rust-windowing has.

[0xpr03]

This is blocked by a mac implementation (possibly just another cfg with its error no) which I can't do as I don't have any apple machine.

[JohnTitor]

Looks reasonable to me, though I also don't have a macOS machine :/ Could we check it with a (temporary) test on CI?

[0xpr03]

With the help of a friend I've come to the following conclusion: There doesn't seem to be an watch-files limit on mac. And if there would be, you would not get any userland reports.. Also the only real limits seems to be how many watchers you can spawn in parallel until you reach a thread limit, that is reported as "Resource temporarily unavailable" which I think is good enough.

Thus I'd only add the linux change and leave everything else as is.