Change exit code to 2 when config file doesn't exist (#1361)

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.

Co-authored-by: Toby Fleming <tobywf@users.noreply.github.com>

black.py

@@ -394,7 +394,7 @@ def target_version_option_callback(
 @click.option(
     "--config",
     type=click.Path(
-        exists=False, file_okay=True, dir_okay=False, readable=True, allow_dash=False
+        exists=True, file_okay=True, dir_okay=False, readable=True, allow_dash=False
     ),
     is_eager=True,
     callback=read_pyproject_toml,

tests/test_black.py

@@ -1645,6 +1645,16 @@ def test_blackd_main(self) -> None:
                 raise result.exception
             self.assertEqual(result.exit_code, 0)
 
+    def test_invalid_config_return_code(self) -> None:
+        tmp_file = Path(black.dump_to_file())
+        try:
+            tmp_config = Path(black.dump_to_file())
+            tmp_config.unlink()
+            args = ["--config", str(tmp_config), str(tmp_file)]
+            self.invokeBlack(args, exit_code=2, ignore_config=False)
+        finally:
+            tmp_file.unlink()
+
 
 class BlackDTestCase(AioHTTPTestCase):
     async def get_application(self) -> web.Application: