test-mime-data: fail when an error occurs

This change should prevent test file omissions like the one fixed in

Always print summary. The condition (error > 0 || failed > 0) was
incomplete: xmatch > 0 should have been checked as well.
shared-mime-info's test_mime.sh passes a list with many expected
failures to test-mime-data, so the condition failed > 0 is always true.
Therefore printing the summary unconditionally makes no difference,
unless there are other users of test-mime-data that don't have expected
failures in their test case lists.
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