make more clear when some rules are not taken into account

[edwindj]

Currently errorlocate issues a warning that some of the rules are skipped, because they are not record-based or non-linear.

A novice user should be warned more clearly, that this can happen.
Suggestion: add an extra function argument: skip_rules = FALSE, that stops the execution when FALSE and otherwise generates a warning.

Thanks Guido van den Heuvel for the suggestion