Return generic validation error were no `Message` key is defined

If we define a validation rule with the message then if the rule fails we can return the message into a Flash element. If we return false the validationErrors array is still there but instead of including a message it includes the key of the rule that failed.
We are going to find if the array contains a rule key instead of a message and we will replace it with a generic message, like:

_field_name_ failed