bisect-kit: handle too much skip properly

BUG=b:127452142
TEST=unittest

Change-Id: Id56c150204d3b3463fdf29917cb2bed4b0e9ee4e
Reviewed-on: https://chromium-review.googlesource.com/1516893
Commit-Ready: Kuang-che Wu <kcwu@chromium.org>
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Chi-Ngai Wan <cnwan@google.com>
diff --git a/bisect_kit/cli_test.py b/bisect_kit/cli_test.py
index 1f630a4..f96cfd1 100644
--- a/bisect_kit/cli_test.py
+++ b/bisect_kit/cli_test.py
@@ -276,8 +276,10 @@
     bisector.main('run', '-1')
     self.assertEqual(bisector.strategy.get_range(), (20, 30))
 
-    bisector.main('skip', '20*10')
-    bisector.main('skip', '30*10')
+    with self.assertRaises(errors.UnableToProceed):
+      bisector.main('skip', '20*10')
+    with self.assertRaises(errors.UnableToProceed):
+      bisector.main('skip', '30*10')
     self.assertEqual(bisector.strategy.get_range(), (20, 30))
 
   def test_cmd_switch(self):