Add better logging when failing to load a test.

typ/tests/runner_test.py

 # Copyright 2014 Dirk Pranke. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
 #
 #    http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 80 matching lines @@
             h.chdir(tmpdir)
             h.write_text_file('load_test.py', d("""\
                 import unittest
                 def load_tests(_, _2, _3):
                     assert False
                 """))
             test_set = TestSet()
             test_set.parallel_tests = [TestInput('load_test.BaseTest.test_x')]
             r = Runner()
             r.args.jobs = 1
-            ret, _, _ = r.run(test_set)
+            ret, _, trace = r.run(test_set)
             self.assertEqual(ret, 1)
+            self.assertIn('Failed to load "load_test.BaseTest.test_x" in '
+                          'run_one_test',
+                          trace['traceEvents'][0]['args']['err'])
         finally:
             h.chdir(orig_wd)
             if tmpdir:
                 h.rmtree(tmpdir)
 
 
 class TestWinMultiprocessing(TestCase):
     def make_host(self):
         return Host()
 
@@ skipping 99 matching lines @@
         self.assertIn('No tests to run.', out)
         self.assertEqual(err, '')
 
 
 class ContextTests(TestCase):
     def test_context(self):
         # This test is mostly intended to be called by
         # RunnerTests.test_context, above. It is not interesting on its own.
         if self.context:
             self.assertEquals(self.context['foo'], 'bar')