Uploader: fix check for error message send from async-upload on failed...

Uploader: fix check for error message send from async-upload on failed uploads, improve styling it a bit, props trepmal, SergeyBiryukov, fixes #20445

git-svn-id: https://core.svn.wordpress.org/trunk@20580 1a063a9b-81f0-0310-95a4-ce76da25c4cd