UI: Avoid using newlines in error messages

Some errors include HTML links directing users to e.g. driver updates
or further information. Using a raw newline instead of <br> causes Qt to
skip parsing the HTML, resulting in an ugly mess of HTML displayed to
the user instead of the intended links.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f