fix template bug that made hgweb break.
problem found by Shun-ichi Goto <shunichi.goto@gmail.com>.
this fix tested with command template and hgweb templates.
# repo.py - repository base classes for mercurial
#
# Copyright 2005 Matt Mackall <mpm@selenic.com>
#
# This software may be used and distributed according to the terms
# of the GNU General Public License, incorporated herein by reference.
class RepoError(Exception): pass