tests: make run-tests exit non-zero if there are "errors"

Previously, if there was an error such as a broken .t file that caused
run-tests.py to encounter an exception during parsing, the test would be
considered in an "errored" state, which is separate from "failed".

The check for whether to exit non-zero or not was based entirely on whether
there were any tests in a "failed" state, so if there was only an error,
run-tests would exit with 0. Our test infrastructure would then consider the
test as passing, causing us to have some tests with false negatives that have
gone undetected for a few weeks now.

Differential Revision: https://phab.mercurial-scm.org/D6452

tests/run-tests.py

@@ -2912,7 +2912,7 b' class TestRunner(object):'
 
                 result = runner.run(suite)
 
-            if result.failures:
+            if result.failures or result.errors:
                 failed = True
 
             result.onEnd()