Tactics should signal wrong ProofInfo

[Engreyight]

Certain tactics, such as mlRewriteBy only work if the goal is using AnyReasoning. However, if they are used with something different, they fail with a useless error message (mlRewriteBy says "not in proof mode"). Instead, they should signal that the ProofInfo is incorrect, or add an extra ProofInfoLE AnyReasoning i goal, like mlApplyMeta does in similar situations.

This limitation is also undocumented.

[berpeti]

There are two subtasks here:

• Update documentation.
• Fix error messages.