With the test view, you can test the look and feel and
validation behavior of the field. Fill in the field and press the
Test button to test it.
You will now see the test form again. If your input was
successfully validated by the field, you will see a message Test
successful: above the test form, followed by the result of the
validation processing (this may be empty text in case you filled
in nothing).
If the field could not be validated, you will see the message There was a
validation error: instead, followed by a description of the error:
field_id gives the field name.error_key is the name of the error message; you can find it
in the messages tab.error_text is the actual text associated with the
'error_key'; you can modify it in the messages tab.