Handle exceptions from checks

The original check code in Nova behaves this way, so for consistency
do it here too.

oslo_upgradecheck/tests/test_upgradecheck.py

@@ -79,3 +79,11 @@ class TestMain(base.BaseTestCase):
             inst = TestCommands()
             result = upgradecheck.main(inst.check)
             self.assertEqual(upgradecheck.UpgradeCheckCode.FAILURE, result)
+
+    def test_main_exception(self):
+        def raises():
+            raise Exception('test exception')
+        mock_argv = ['test-status', 'upgrade', 'check']
+        with mock.patch.object(sys, 'argv', mock_argv, create=True):
+            result = upgradecheck.main(raises)
+            self.assertEqual(255, result)

oslo_upgradecheck/upgradecheck.py

@@ -16,6 +16,7 @@
 import functools
 import sys
 import textwrap
+import traceback
 
 import enum
 from oslo_config import cfg
@@ -148,4 +149,9 @@ def main(check_callback):
     conf.register_cli_opt(opt)
     conf(sys.argv[1:])
 
+    try:
         return conf.category.action_fn()
+    except Exception:
+        print(_('Error:\n%s') % traceback.format_exc())
+        # This is 255 so it's not confused with the upgrade check exit codes.
+        return 255