beaker-expire-distros: distinguish missing trees from server errors

Change-Id: Id76f09b6adc7dcd81383b59eb204b12aedbd7036
beaker-expire-distros: fix handling nfs+iso URLs

    Error checking for existence of URL nfs+iso://example.com:/srv/pub/RHEL-7.0-20120711.2-Server-x86_64-iso/ for distro tree 958:
    Cannot access NFS server o or autofs not running (/net/o does not exist)

Change-Id: I3b833ba1e88447e6cebd02ec310c2b4efb5afc32
Merge changes I3b833ba1,Id76f09b6 into release-0.12

* changes:
  beaker-expire-distros: fix handling nfs+iso URLs
  beaker-expire-distros: distinguish missing trees from server errors