bad appendpath policies are ugly

 affects bzr
 status confirmed
 importance medium
 done

Setting 'push_location:policy = badvalue' generates:
  File "/home/robertc/source/baz/bzr-test-fixes/bzrlib/config.py", line
733, in _get_user_option
    value = source()._get_user_option(option_name)
  File "/home/robertc/source/baz/bzr-test-fixes/bzrlib/config.py", line
409, in _get_user_option
    policy = self._get_option_policy(section, option_name)
  File "/home/robertc/source/baz/bzr-test-fixes/bzrlib/config.py", line
602, in _get_option_policy
    return _policy_value[policy_name]
KeyError: u'badvalue'

We would be better of listing the allowed values at this point.