allow threads to die with ExitSuccess without printing a message

[mitchellwrosen]

Overview

This PR just whitelists ExitSuccess so that background threads that unexpectedly die with ExitSuccess don't cause a message like

Uh oh, an unexpected exception brought the process down! That should never happen. Please file a bug report.

Here's a stringy rendering of the exception:

  ExitSuccess

This isn't an awesome solution – throwing ExitSuccess from a background thread isn't an established practice or anything. But we simply don't have control over these threads, since they're deep within the lsp library.

A better solution would be to patch lsp to have better thread hygiene, but this solution is a one-liner and seems pragmatic, for now.

Test coverage

I tested this change manually

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closes #4737