"unknown branch format" for unknown repository

If you have a pack-0.92 repository, doing "bzr status" with an older version
will give a slightly bogus statement:

% ~/dev/bzr/releases/bzr.0.91/bzr status
bzr: ERROR: Unknown branch format: 'Bazaar pack repository format 1 (needs bzr
0.92)\n'

It should be:

Unknown repository format: ...

I think it is because we have "UnknownFormatError" which just always says "branch".

The easy fix would be to just have it take an optional "kind" parameter, which
would be set to 'branch' by default, and could be set to 'repository' or
'working tree' at the correct time.