Ensure it is valid to invoke an action
public String validate<actionName>([<parameter type> param]...)
Complements an action method.
This validate method is used to check that the actions parameters are all valid and that the
action method can be safely invoked, that is, if it is invoked that it will not fail. If a String object
is returned then the action should not be invoke. The string itself will then be made visible to user
to inform them why the method cannot be invoked. If the method returns null then the action
method will be invoked with the same set of parameters.