Rescheduler: don't silently stop when errors are encountered

[tkluck]

This was helpful during work on #443 to make sure my typos didn't pass silently. Maybe it's helpful for others too?

These thunks should be quite reliable (e.g., they don't contain user-specified code) but during development it's nice to have visibility on errors and keep the background workers functioning.

[timholy]

Probably a good idea, I'm interested to see what issues this might catch. You'll have to handle some circumstances, like file deletion, specially.

[tkluck]

The CI failures are only on julia-nightly and are caused by JuliaLang/julia#34660 . I think this PR is probably good to merge.

[tkluck]

You'll have to handle some circumstances, like file deletion, specially.

Wait, maybe I missed what you were trying to say here. You mean that some of the Reschedulers actually should silently exit if an error occurs? I completely missed that. Could you give me a pointer?

[timholy]

The failures are only on nightly

Check travis, you'll see lots of

┌ Warning: [Revise] Error during a background task
│   err = IOError: FileMonitor (start): no such file or directory (ENOENT)
└ @ Revise ~/build/timholy/Revise.jl/src/types.jl:249

The tests actually pass but that seems to cause Travis to fail to realize that, so basically all tests are marked as failing. I can't really merge this under those circumstances.

[tkluck]

Ah thanks for pointing that out! That was my confirmation bias at work. Let me see what I can do.

[tkluck]

I'm sorry, I'm going to close this pull request because I don't think I'll realistically have time to finish it. But I'm very excited that #488 got merged :-)