Added exception for no containers found in directory.
Added exception for no containers found in directory.

When regenerating a container, if no state files are actually found in
the given directory, a NoContainersError is raised.

datreant/datreant,datreant/datreant.core,datreant/datreant,datreant/datreant.data,dotsdl/datreant,andreabedini/datreant,datreant/datreant.core