bisect-kit: lease DUT from skylab

Because autotest lab will be deprecated soon and the transition period
should be short, the code is implemented slightly hacky. After
migration, code for autotest lab will be removed.

BUG=b:140700328
TEST=test manually

Change-Id: I1f579ee91b8057a661c7ad82bbdd866daef64a7a
Reviewed-on: https://chromium-review.googlesource.com/1886555
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Commit-Ready: Kuang-che Wu <kcwu@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@chromium.org>
diff --git a/bisect_kit/errors.py b/bisect_kit/errors.py
index a8fc66c..4cde7fa 100644
--- a/bisect_kit/errors.py
+++ b/bisect_kit/errors.py
@@ -30,6 +30,10 @@
   """Unable to allocate DUT from lab."""
 
 
+class DutLeaseException(Exception):
+  """Issues of skylab DUT lease."""
+
+
 class WrongAssumption(Exception):
   """Wrong assumption.