`MVarId.congr?` and `MVarId.hcongr?` should return `none` if an
exception is thrown while applying congruence theorem.
`MVarId.hcongr?` should try `eq_of_heq` before trying to apply
`hcongr` theorem.
closes#1787
BTW: Lean 4 `congr` tactic is applying `assumption`. Lean 3 version does not.
