getdeps: introduce TransientFailure exception type

Summary:
The goal is to return an error code > 127 in the case of a
transient, retryable, infrastructure error.  This diff generates
those in the case of failure in downloading a URL or from interacting
with LFS.

Reviewed By: strager

Differential Revision: D15266838

fbshipit-source-id: 4f52a791320123968869032c37912dded464a86e

build/fbcode_builder/getdeps/errors.py

@@ -0,0 +1,16 @@
+# Copyright (c) 2019-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree. An additional grant
+# of patent rights can be found in the PATENTS file in the same directory.
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+
+class TransientFailure(Exception):
+    """ Raising this error causes getdeps to return with an error code
+    that Sandcastle will consider to be a retryable transient
+    infrastructure error """
+
+    pass