resource hasn't been locked with, we should return an error
instead of a 20X response.  See
http://lists.w3.org/Archives/Public/w3c-dist-auth/2001JanMar/0099.html
for rationale.

Prior to Zope 2.11, we returned a 204 under this circumstance.
We choose do what mod_dav does, which is return a '400 Bad
Request' error.

This was caught by litmus locks.notowner_lock test #10.