Clean up the document load/save error dialog

This PR proposes to fix some issues I found with the current layout of DlgLoadMessages.

• The rich text details box is always shown, irrespective of whether there are actual error messages or not.
• The layout is done completely by hand, but the window is semantically a warning dialog, so it should, in my opinion, follow the operating system conventions.

I've fixed it by changing the base class over to QMessageBox, and setting each piece as follows:

• Error message: text
• The "Reason": informativeText
• The contents of the errors/warnings: detailedText

And the last element is fixed by doing QString::split but skipping the empty parts; previously, the error and warning strings that were empty resulted in a QStringList with two elements, while it should have been an empty set.

The most relevant bit, I think, is that the error detail is no longer HTML list styled as Qt doesn't provide support for that.

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Test Plan

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• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
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• I made sure my code conforms to the standards set in the HACKING file.
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