web: Render external service form errors/warnings as Markdown
Created by: mrnugget
This is a really simple first fix for #4842, as suggested by @felixbecker.
The proper solution would probably be to make the errors themselves more readable & easier to parse, but that's a lot more work than this fix here.
Until we find the time/muse to do that, this here already improves on the current sate.
It now looks like this:
