Implement more beautiful error handling when a mac is already in use
Implement more beautiful error handling when a mac is already in use

References #288
agdsn/pycroft,lukasjuhrich/pycroft,agdsn/pycroft,lukasjuhrich/pycroft,lukasjuhrich/pycroft,agdsn/pycroft,agdsn/pycroft,lukasjuhrich/pycroft,agdsn/pycroft