Fold RevertFileAction class into RevertAction

Fold RevertFileAction class into RevertAction

RevertAction was simply a dummy class for outputting an error message
if the action was not done in the file namespace. I've instead
moved RevertFileAction to RevertAction and threw an exception
from within checkCanExecute().

Change-Id: I7b87de60680009bf4e74d33342cbe81cc0d211b5