#7930 Eliminate Validator.isValid()

#7930 Eliminate Validator.isValid()

Eliminate the redundant method Validator.isValid(). Validator.validate()
does the same but throws an exception including the validation error
message.

Migration:
- replace implementations of isValid() with internalIsValid() in custom
subclasses of AbstractValidator
- use Validator.validate() and catch exceptions rather than call
Validator.isValid()
- note that AbstractField still provides an isValid() method that is
internally based on validate()