The `decide` tactic produces error messages that users find to be
obscure. Now:
1. If the `Decidable` instance reduces to `isFalse`, it reports that
`decide` failed because the proposition is false.
2. If the `Decidable` instance fails to reduce, it explains what
proposition it failed for, and it shows the reduced `Decidable` instance
rather than the `Decidable.decide` expression. That expression tends to
be less useful since it shows the unreduced `Decidable` argument (plus
it's a lot longer!)
Examples:
```lean
example : 1 ≠ 1 := by decide
/-
tactic 'decide' proved that the proposition
1 ≠ 1
is false
-/
opaque unknownProp : Prop
open scoped Classical in
example : unknownProp := by decide
/-
tactic 'decide' failed for proposition
unknownProp
since its 'Decidable' instance reduced to
Classical.choice ⋯
rather than to the 'isTrue' constructor.
-/
```
When reporting the error, `decide` only shows the whnf of the
`Decidable` instance. In the future we could consider having it reduce
all decidable instances present in the term, which can help with
determining the cause of failure (this was explored in
8cede580690faa5ce18683f168838b08b372bacb).
