refactor(compute/serve): replace log.Fatal usage with channel

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[awilliams-fastly]

Similar comment as this #1082 (comment).

Otherwise, LGTM

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Thanks @awilliams-fastly I think that makes sense for the other PR but for this one I think there's a wrinkle where we have...

eventHandler := func(modifiedFile string, _ fsnotify.Op) { ... } 

...which can't return an error in its current form and I can't convert it to return an error because it's contained by another function debounced(func() {}) that we don't control and can't convert to handle an error. So it gets harder to reason about at that point.

So I think I'll leave it as is for this PR but for I'll definitely look at doing your suggestion for the other PR.