Fix class-not-found error message, add nodename

Signed-off-by: martin f. krafft <madduck@madduck.net>
Recurse classes defined in class_mappings

Class mappings produce a list of classes. Obviously, this list needs to
be iterated and each class recursed too.

Closes: https://github.com/madduck/reclass/issues/16
Signed-off-by: martin f. krafft <madduck@madduck.net>