[recipe_modules/file] add real file module to engine.

recipe_modules/file/examples/error.py

Index: recipe_modules/file/examples/error.py
diff --git a/recipe_modules/file/examples/error.py b/recipe_modules/file/examples/error.py
new file mode 100644
index 0000000000000000000000000000000000000000..e625fe747dcd56aa9c445231654071d0f8d39ea6
--- /dev/null
+++ b/recipe_modules/file/examples/error.py
@@ -0,0 +1,24 @@
+# Copyright 2017 The LUCI Authors. All rights reserved.
+# Use of this source code is governed under the Apache License, Version 2.0
+# that can be found in the LICENSE file.
+
+DEPS = [
+  "file",
+  "path",
+]
+
+
+def RunSteps(api):
+  try:
+    api.file.read_text(
+      'does not exist', api.path['start_dir'].join('not_there'))
+    assert False, "never reached"  # pragma: no cover
+  except api.file.Error as e:
+    assert e.errno_name == 'ENOENT'
+
+
+def GenTests(api):
+  yield (
+    api.test('basic')
+    + api.step_data('does not exist', api.file.errno('ENOENT'))
+  )