Add permission check for device file

[krichter722]

Adding a permission check for the specified device file would improve the feedback in case the permissions to rw are missing, e.g. python pykmaze.py --port=/dev/ttyUSB0 -i fails with

No pyftdi
Error: Cannot open device "/dev/ttyUSB0"

if the user isn't member of the dialout group which owns /dev/ttyUSB0 by default. The feedback ~Missing read/write permissions for [device file]. This is usually fixed by adding the user to the dialout group`. is much more helpful. This would answer #1.

experienced with 0ceab85