Allow errors in validator_definition elements

This change allows `errors` in ```validator_definition``` elements in ```validators.xml``` files. This means validator definitions can define default error messages instead of providing those on each concrete validator defined later on (based on the validator definition).

Example from e.g. ```app/config/validators.xml``` file:
```xml
        <validator_definitions>
                
            <validator_definition name="custom" class="SomeCustomValidator">
                <ae:parameters>
                    <ae:parameter name="required">true</ae:parameter>
                </ae:parameters>                
                <errors>
                    <error>This is some default error that wasn't possible before this patch.</error>
                    <error for="special">Some very special error message.</error>
                </errors>
            </validator_definition>
            
        </validator_definitions>
```

This changeset is adapted from a version @dracoblue and me did some time ago. There are no tests as I would need some hints/help on that. Comments welcome.