Ensure that a not-found file produces an error

- this restores a flake8 2.x behaviour that was lost in the refactor

tests/integration/test_checker.py

@@ -216,7 +216,7 @@ def test_report_order(results, expected_order):
     # Create a placeholder manager without arguments or plugins
     # Just add one custom file checker which just provides the results
     manager = checker.Manager(style_guide, [], [])
-    manager.checkers = [file_checker]
+    manager.checkers = manager._all_checkers = [file_checker]
 
     # _handle_results is the first place which gets the sorted result
     # Should something non-private be mocked instead?

tests/integration/test_main.py

@@ -195,3 +195,12 @@ ignore = F401
     py_file.write_text(u"import os\n")
 
     _call_main(["--isolated", "--config", str(config), str(py_file)], retv=1)
+
+
+def test_file_not_found(tmpdir, capsys):
+    """Ensure that a not-found file / directory is an error."""
+    with tmpdir.as_cwd():
+        _call_main(["i-do-not-exist"], retv=1)
+    out, err = capsys.readouterr()
+    assert out.startswith("i-do-not-exist:0:1: E902")
+    assert err == ""