https://bugs.webkit.org/show_bug.cgi?id=96453

Reviewed by Tony Chang.

Add a special error message when a configuration file is missing.

Also update the help message of --source-json-path to signify the fact it specifies
the configuration file on performance tests bots.

* Scripts/webkitpy/performance_tests/perftestsrunner.py:
(PerfTestsRunner._parse_args):
(PerfTestsRunner._generate_and_show_results):
(PerfTestsRunner._merge_slave_config_json):
* Scripts/webkitpy/performance_tests/perftestsrunner_unittest.py:
(_test_run_with_json_output): Make upload to fail by default and assert the result
in the function so that we can return logs instead.
(_test_run_with_json_output.mock_upload_json):
(test_run_with_json_output):
(test_run_with_description):
(test_run_respects_no_results):
(test_run_with_slave_config_json):
(test_run_with_bad_slave_config_json):
(test_run_with_multiple_repositories):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@128511 268f45cc-cd09-0410-ab3c-d52691b4dbfc