Root class of a Validator which is capable of being parsed from a
compact String representation. If it does not implement any further
mixins, it is assumed to be a Spring validator requires no argument.
That is, a validator which is either present or absent on a field.
author: Antranig Basman (antranig@caret.cam.ac.uk)
Returns the priority of the validator. If there are multiple potential
validation failures, only those with the lowest priority number
will be reported.