Validator::formatMessage: support dynamic error messages

[jtojnar]

This is useful when you want to show user an informative error message, whose text depends on some external data that depends on the value of some other field.

For example, my form allows user to choose a category of a team. Each category has a different limit for number of allowed members, which is specified in config.neon. I want to show the user the limit as part of the error message when she manages to exceed the limit.

• new feature
• BC break? no
• doc PR: none yet