bisect-kit: handle DUT lost connection properly

BUG=b:126074485
TEST=run diagnose_cros_autotest.py manually

Change-Id: I28cd60d0a74b61b6d1dbe4b2b30f36f4ff039df5
Reviewed-on: https://chromium-review.googlesource.com/1496683
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/errors.py b/bisect_kit/errors.py
index 501b687..7cc0f64 100644
--- a/bisect_kit/errors.py
+++ b/bisect_kit/errors.py
@@ -91,3 +91,7 @@
 
   Like configuration errors, network errors, DUT issues, etc.
   """
+
+
+class SshConnectionError(Exception):
+  """SSH connection error."""