py.test also can generate "errors"

Here is e.g. corresponding summary:

    ===== 12 failed, 14 passed, 4 xfailed, 12 error in 19.58 seconds =====

nxdtest

@@ -372,7 +372,7 @@ class PyTest:
         stat_set('failure_count', 'failed')
         stat_set('expected_failures', 'xfailed')
         stat_set('unexpected_successes', 'xpassed')
-        # don't set error_count - everything goes to failure_count
+        stat_set('error_count', 'error')
         stat['test_count'] += get('passed',  0)
         return stat