1. Prevent duplicate XML error to show up if the __MACOSX error is already present.
2. Add a general "Use the command-line tools" message on the template if any error is present.
3. Use <strong> to highlight some elements of the error messages.
This is marginally related to: #175.
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TODO: I could not find a way to also have <strong> for missing_wheel and forbidden_filepaths.
My idea is that we could sanitize their parameters first, and then combine with the mark_safe()
error message.
Feel free to take the patch over and finalize it.
