Exposed :key (in addition to :message)

I have found use case where I could do with the key of the error array exposed (proposing :key).

If I echo out my errors, on some occasions I output the errors in a single block then treat them using JS (usually moving them to an element on the page by the ID stored in a data attribute).

If I want to print <li data-key=":key">:message</li>, I cannot currently access the :key part (where :key is the ID of the associated field).

Being able to do this (as adjusted) allows a user to identify exactly what field an individual message was associated with e.g. when using ->all(), and is a simple exposure of data already held in the object.

Example:

Format: 
    
    <li data-key=":key">:message</li>

    <ul class="errors">
        <li data-key="item_firstname">You didn't enter your first name</li>
    </ul>