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Fix check in import to check if definition is inside descripGrp
Check in import if xref tag is child of langSet

This allows to avoid dealing with xref that are at lower levels, for
example inside a descripGrp alongside a definition in order to provide
the source for that definition.
Change should in favor of must in several import error messages
Split up several import error messages

This allows to reduce indents, and to provide shorter messages for translators