Change exit code to 2 when config file doesn't exist

[tobywf]

Fixes #1360, where an invalid config file causes a return/exit code of 1. This change means this case is caught earlier, treated like any other bad parameters, and results in an exit code of 2.

Unit test fails before change is made:

======================================================================
FAIL: test_invalid_config_return_code (tests.test_black.BlackTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "<snip>/black/tests/test_black.py", line 1654, in test_invalid_config_return_code
    self.invokeBlack(args, exit_code=2, ignore_config=False)
  File "<snip>/black/tests/test_black.py", line 162, in invokeBlack
    self.assertEqual(result.exit_code, exit_code, msg=runner.stderr_bytes.decode())
AssertionError: 1 != 2 : Error: Could not open file <snip>/blk_h9d894at.log: Error reading configuration file: [Errno 2] No such file or directory: '<snip>/blk_h9d894at.log'

[tobywf]

Please let me know if there's anything you need from me to get this merged (did I forget a label or an assignee?). Would be nice to get it in before the refactor happens :)

[zsol]

Thanks! :)