This PR adds an explanation to the error message when `cases` and
`induction` are applied to a term whose type is not an inductive type.
For `Prop`, these tactics now suggest the `by_cases` tactic. Example:
```
tactic 'cases' failed, major premise type is not an inductive type
Prop
Explanation: the 'cases' tactic is for constructor-based reasoning as well as for applying
custom cases principles with a 'using' clause or a registered '@[cases_eliminator]' theorem.
The above type neither is an inductive type nor has a registered theorem.
Consider using the 'by_cases' tactic, which does true/false reasoning for propositions.
```
[Zulip
discussion](https://leanprover.zulipchat.com/#narrow/channel/270676-lean4/topic/Improving.20the.20error.20for.20.60cases.20p.60.20when.20.60p.60.20is.20a.20proposition/near/488882682)
