bisect-kit: use more descriptive exception message

Exceptions with technical detail is moved to log.

BUG=b:124841028
TEST=None

Change-Id: I61df24b69f926807b52c96de6dd9b184e8cc3a21
Reviewed-on: https://chromium-review.googlesource.com/1506333
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@chromium.org>
Reviewed-by: Chi-Ngai Wan <cnwan@google.com>
diff --git a/bisect_kit/errors.py b/bisect_kit/errors.py
index 7cc0f64..a8fc66c 100644
--- a/bisect_kit/errors.py
+++ b/bisect_kit/errors.py
@@ -44,6 +44,14 @@
   """
 
 
+class DiagnoseContradiction(Exception):
+  """Contradiction happened during diagnose.
+
+  Test result of individual component/version is unreliable/untrustable
+  (something wrong and/or flakiness out of control).
+  """
+
+
 class VerificationFailed(Exception):
   """Bisection range is verified false."""