Override Drupal.theme.message to to make sure JS messages get rendered correctly

3 small tests on the messages... I hope it is inline with what is asked:

test1: If I clear the caches, I can see a message (cache cleared) with a closing button=>OK
test2: If I add an error in the code $form['this_key_do_not_exists']='error'; I can see a message without closing button and with the text which extends outside the dark background (Impossible to read it as it is white on white) => NOT OK
test3: If I create an element validation error (Ex: upload an xls in a widget asking for jpg) then there are NO message, which is really bad => NOT OK => see #3157576: messages color not set when error on file upload for more info on test3

test2: If I add an error in the code $form['this_key_do_not_exists']='error'; I can see a message without closing button and with the text which extends outside the dark background (Impossible to read it as it is white on white) => NOT OK

Can you provide more clear steps and explanations about what and where and how to reproduce?

test3: If I create an element validation error (Ex: upload an xls in a widget asking for jpg) then there are NO message, which is really bad

Should be fine now. I decided to not override file.js to change messages markup. So let's keep messages for file widget without close button. Check the following styles below, this is how error messages looks right now for file widget:

Reroll