Report an error when iIntros fails to find a forall

The error handling for iIntros (?) and similar tactics didn't correctly report failures when the goal couldn't be turned into a universal quantifier. This is something missing from !482 (merged) due to no test triggering the error.

Great. Thanks!

merged

mentioned in commit 4ecceb97

changed title from Report an error when iIntro fails to find a forall to Report an error when iIntros fails to find a forall

changed the description