Improve exception for wrong locales
I created a locale with the identifier "en_EN". I took me a while to realize that this is an invalid locale of course, as there is no country identified by "EN". The error message you'd currently see on trying this is the following:
#1328879703: The path "resource://TYPO3.FLOW3/Private/I18n/CLDR/Sources/main/en_EN.xml" does not point to an existing and accessible XML file.
We could improve this by throwing a more specific exception telling what is actually wrong - "EN is not a valid region code according to ISO xyz / the CLDR".