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 updates the formatting of, and adds explanations for, "unknown
identifier" errors as well as "failed to infer type" errors for binders
and definitions.
It attempts to ameliorate some of the confusion encountered in #1592 by
modifying the wording of the "header is elaborated before body is
processed" note and adding further discussion and examples of this
behavior in the corresponding error explanation.
