FIX: Call `_clearFlash()` when displaying a modal (PR #14848)

d-modal-body.js was setting the text of a modal-alert element to "", but not removing any classes on that element. Changing this to call _clearFlash() ensures that a variety of styling classes are also removed from the element, which prevents empty alert elements being included on any subsequent modals that are displayed.

Several other controllers have also been modified to change the class of the error from alert-error to error. The alert-is unnecessary, as it is added by_flash(msg)withind-modal-body.js`.


@martin-brennan Can you double-check this doesn’t trample on any of your recent accessibility changes?

I don’t think so, it only is dealing with CSS classes that don’t matter for a11y, and the _clearFlash function appears to do the same thing but with the class removal too :+1: