Description of problem:
With LVM plugin enabled, I can't scrub traditional directories


Version-Release number of selected component (if applicable):
$ rpm -q mock
mock-1.2.0-1.fc20.noarch


How reproducible:


Steps to Reproduce:
1. Init some build root
2. Enable LVM plugin
3. mock -r epel-5-x86_64 --scrub all

Actual results:
$ mock -r epel-5-x86_64 --scrub all
INFO: mock.py version 1.2.0 starting (python version = 2.7.5)...
Start: init plugins
INFO: selinux enabled
Finish: init plugins
Start: run
Start: scrub ['all']
finishing: scrub ['all']
Finish: scrub ['all']
ERROR: (0, '')
Traceback (most recent call last):
  File "/usr/sbin/mock", line 809, in <module>
    main()
  File "/usr/lib/python2.7/site-packages/mockbuild/trace_decorator.py", line 84, in trace
    result = func(*args, **kw)
  File "/usr/sbin/mock", line 647, in main
    run_command(options, args, config_opts, commands, buildroot, state)
  File "/usr/lib/python2.7/site-packages/mockbuild/trace_decorator.py", line 84, in trace
    result = func(*args, **kw)
  File "/usr/sbin/mock", line 669, in run_command
    commands.scrub(options.scrub)
  File "/usr/lib/python2.7/site-packages/mockbuild/trace_decorator.py", line 84, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.7/site-packages/mockbuild/backend.py", line 83, in scrub
    self.plugins.call_hooks('scrub', scrub)
  File "/usr/lib/python2.7/site-packages/mockbuild/trace_decorator.py", line 84, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.7/site-packages/mockbuild/plugin.py", line 65, in call_hooks
    hook(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/mockbuild/plugins/lvm_root.py", line 318, in hook_scrub
    pool = vg.lvFromName(self.pool_name)
LibLVMError: (0, '')


Expected results:
Scrub succeeds or at least provide some sensible error message.


Additional info: