This PR improves the error messages produced by the `split` tactic,
including suggesting syntax fixes and related tactics with which it
might be confused.
Note that, to avoid clashing with the new error message styling
conventions used in these messages, this PR also updates the formatting
of the message produced by `throwTacticEx`.
Closes#6224
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)
