Support warn messages

[fwendland]

MARK currently support only error messages. When a check fails, an error is produced and the corresponding message can be printed.

In some cases, a rule may not fail outright but indicate a warning. For example, a certain pattern while correct may be discouraged. Or a rule checks a property that has an expiration date. Thus, this rule may want to print a warning when it approaches the expiration date.

To this end, MARK should be extended by some ability to generate warnings.