bisect-kit: fix exception in view and log subcommand if 'init' failed

BUG=b:160228376
TEST=diagnose_cros_tast.py view and log (--json)

Change-Id: I45621c5d208320cfa91ee5354f4881d22a454b30
diff --git a/bisect_kit/errors.py b/bisect_kit/errors.py
index 43d3111..893bb6e 100644
--- a/bisect_kit/errors.py
+++ b/bisect_kit/errors.py
@@ -19,6 +19,10 @@
     return self.message
 
 
+class Uninitialized(Exception):
+  """'init' must succeed before other subcommands."""
+
+
 class ExecutionFatalError(Exception):
   """Fatal error and bisect should not continue.