Reject return in pattern guards and if conditions

[yannbolliger]

Currently (#108), there are two negative test cases for these two things. But the test cases don't "fail" they just let
Stainless crash. We want to disallow the two cases already in the Rust frontend and hence fail_extraction
instead of crash_verification.

(At the same time, one could again remove the crash_verification flag, as it's a bit ugly to let the backend crash.)