Code review comment for lp:~thumper/launchpad/lp-distroseries

A relatively simple branch that addresses bug 480024.

tests:
  TestExpandURL

QA:
  bzr revno lp:ubuntu/karmic
or
  bzr revno lp://dev/ubuntu/warty

Gets a nice error like:
$ bzr revno lp://dev/ubuntu/warty
bzr: ERROR: Invalid url supplied to transport: "lp://dev/ubuntu/warty": Warty is a distroseries, and a distroseries doesn't have a default branch.

instead of an oops.