Printing unknown reason

[Halbaroth]

We should support the SMT-LIB syntax (get-info :reason-unknown) to be able to print the reason why Alt-Ergo answers timeout.

See #829 (comment)

The PR #837 partially solves this issue.

[Stevendeo]

I have been testing something before closing this issue, and something annoying happens.

• If we set a global timeout, we stop right after the timeout so the get-info :reason-unknown is not reached
• If we set a timeout per goal and return unknown, (get-info :reason-unknown) returns an error.

[Stevendeo]

Ah, actually with the latest next (after Fix timeout) it is worse with timelimit-per-goal. As the assertion fails, it returns a first unknown before reaching check-sat which also returns unknown...

[bclement-ocp]

Works now.