Invalid selector in MaximumFileSizeExceededUploadTest

Problem/Motivation

The '.messages--error' selector doesn't actually appear, but this is obscured by the fact $this->assertSession()->waitForElement doesn't assert the element actually appears.

Proposed resolution

switch to '[aria-label="Error message"]' and wrap the assertion in $this->assertNotEmpty().

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet