Add withException, similar to safe-exceptions and unliftio. Closes #304

[torgeirsh]

I copied and edited the code from unliftio. Since other functions from Effectful.Exception are used, the withRunInIO in the original code isn't needed, but I don't know if it would be better to use reallyUnsafeUnliftIO and call the Control.Exception functions directly. I'm not an expert on exceptions, so I don't know if it affects the safety.

For reference, safe-exceptions has a slightly different implementation that uses generalBracket.

[arybczak]

Superseded by #311.

[torgeirsh]

Thanks! I noticed that the replacement doesn't use masking. Isn't there a risk of losing the original exception if a new asynchronous exception occurs between the catch and the rethrow?

[arybczak]

the replacement doesn't use masking

It does, catch masks the exception handler. It uses mask rather than uninterruptibleMask, but that's a tradeoff that's discussed here if you're interested in details.

Effectful.Exception is supposed to be a thin wrapper for Control.Exception so it doesn't use uninterruptibleMask, but if someone really wants the behavior of safe-exceptions, the library works just fine with effectful ;)