Making ref range failure messages more explicit.

scripts/slave/recipe_modules/auto_bisect_staging/local_bisect.py

Index: scripts/slave/recipe_modules/auto_bisect_staging/local_bisect.py
diff --git a/scripts/slave/recipe_modules/auto_bisect_staging/local_bisect.py b/scripts/slave/recipe_modules/auto_bisect_staging/local_bisect.py
index 0a511704a9f5ca98a9e6c8a6d50da2e107e3270f..36438e0665d60330fe109f9dadb79b34c73adbf5 100644
--- a/scripts/slave/recipe_modules/auto_bisect_staging/local_bisect.py
+++ b/scripts/slave/recipe_modules/auto_bisect_staging/local_bisect.py
@@ -81,12 +81,14 @@ def _gather_reference_range(bisector):  # pragma: no cover
     bisector.surface_result('REF_RANGE_FAIL')
     bisector.failed = True
     raise bisect_exceptions.InconclusiveBisectException(
-        'Testing the "good" revision failed')
+        'Testing the "good" revision failed: ' +
+        bisector.good_rev.failure_reason)
   elif bisector.bad_rev.failed:
     bisector.surface_result('REF_RANGE_FAIL')
     bisector.failed = True
     raise bisect_exceptions.InconclusiveBisectException(
-        'Testing the "bad" revision failed')
+        'Testing the "bad" revision failed: ' +
+        bisector.bad_rev.failure_reason)
 
 def _bisect_main_loop(bisector):  # pragma: no cover
   """This is the main bisect loop.