bisect-kit: support alternative term for 'old' and 'new' behavior

With this change, error messages like
 VerifyOldBehaviorFailed: rev=R80-12737.0.0 expect "old" but got "new" 2 times
becomes easier to understand
 VerifyOldBehaviorFailed: rev=R80-12737.0.0 expect "PASS" but got "FAIL" 2 times

BUG=b:142694486
TEST=unittest

Change-Id: Iae8281125a7d4959d77bcc62cd7c757a4196ec8b
diff --git a/bisect_kit/errors.py b/bisect_kit/errors.py
index 4cde7fa..43d3111 100644
--- a/bisect_kit/errors.py
+++ b/bisect_kit/errors.py
@@ -75,15 +75,17 @@
 class VerifyOldBehaviorFailed(VerificationFailed):
   """Old version does not behave as old."""
 
-  def __init__(self, rev, bad_times=1):
-    super(VerifyOldBehaviorFailed, self).__init__(rev, 'old', 'new', bad_times)
+  def __init__(self, rev, bad_times=1, term_old='OLD', term_new='NEW'):
+    super(VerifyOldBehaviorFailed, self).__init__(rev, term_old, term_new,
+                                                  bad_times)
 
 
 class VerifyNewBehaviorFailed(VerificationFailed):
   """New version does not behave as new."""
 
-  def __init__(self, rev, bad_times=1):
-    super(VerifyNewBehaviorFailed, self).__init__(rev, 'new', 'old', bad_times)
+  def __init__(self, rev, bad_times=1, term_old='OLD', term_new='NEW'):
+    super(VerifyNewBehaviorFailed, self).__init__(rev, term_new, term_old,
+                                                  bad_times)
 
 
 class UnableToProceed(Exception):