Tails Installer should not list read-only media

It has been reported that:

on tails-i386-feature_uefi-0.23-20140110T0926Z-568489c, the
tails installer, at least in Clone&Install mode, will detect my 2nd
USB stick, will start the process and say “Resetting Master Boot
Record of /dev/sdd”, and then just hang if the usb-stick is
read-only.

They suggest:

maybe it should catch that and produce some sane error message.

Parent Task: #7019

Original created by @alant on 6610 (Redmine)