Fix Ember-Data error extraction

Hi,

This PR includes fix that simplify and unify how error message are extracted when using Ember-Data. I've also slightly updated the doc. This also removes the "setCustomError" message on validatable input, as I now take advantage of the "setCustomValidity" to set server errors as custom message, adn take advantage of the native invalid event.

It also removes useless listeners on input.

@swatijadhav as I don't have tests yet I'm not sure this does not cause any regression. I'm tagging as 0.0.11, could you please try it ?