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Reliably report and reject invalid XML files following rP16733, refs #245…

Description

Reliably report and reject invalid XML files following rP16733, refs #245, fixes #5222.

Don't write XMB files for XML files that failed the validation, otherwise the XML validation error will not be reported on consecutive program starts anymore (as the XMB is loaded, skipping validation).
This had resulted in invalid XML going unnoticed and committed in credentials.xml in rP21847.

Differential Revision: https://code.wildfiregames.com/D1574
Reported by: gameboy
Comments By: bb
Tested on: clang 8.0.1, Jenkins