async: extract_exn

[vbmithr]

This is need in order for the Async version to work properly.
Without it, try_with with not return the exception raised by user, but instead an exception wrapper which does not play nice with testing.

[craigfe]

Not being familiar with Async, my understanding is that extract_exn:true discards some backtrace and monitor information that might be helpful for debugging. In what sense does the exception wrapper not play nicely with testing?

[vbmithr]

In the sense that alcotest (apparently) uses exceptions when failing tests and uses the exception to display the problem (i.e. the function returned x instead of y in the case of an int function being tested). If not using extract_exn:true, this information is lost and you just see an unhelpful general exception.

[craigfe]

Added a test case for this in #279.

It's true that Async's exception wrapping prevents exceptions raised by Alcotest.check from being properly displayed at the top-level, and adding extract_exn:true fixes this behaviour. Unfortunately, this also throws away stack trace information from user exceptions in tests (example).

I think a more granular solution is necessary to ensure that only Check_error exceptions are unwrapped in this way.