bisect-kit: fix lint warnings from new lint rules

This also improved python3 compatibility.

BUG=b:140004822
TEST=pylint; unittest

Change-Id: I487e25a055190dd5d4059e8a0c0589170c182dea
diff --git a/bisect_git.py b/bisect_git.py
index 6f09aba..22b48eb 100755
--- a/bisect_git.py
+++ b/bisect_git.py
@@ -43,11 +43,11 @@
     try:
       opts.old = git_util.get_commit_hash(opts.git_repo, opts.old)
     except ValueError as e:
-      raise errors.ArgumentError('--old', e.message)
+      raise errors.ArgumentError('--old', str(e))
     try:
       opts.new = git_util.get_commit_hash(opts.git_repo, opts.new)
     except ValueError as e:
-      raise errors.ArgumentError('--new', e.message)
+      raise errors.ArgumentError('--new', str(e))
 
     config = dict(git_repo=opts.git_repo)
     revlist = git_util.get_revlist(opts.git_repo, opts.old, opts.new)