Say what to do when the serial port is not readable

AccessDenied on /dev/ttyUSB0 is the first thing anyone hits, and "cannot open
/dev/ttyUSB0: AccessDenied" does not help. The steps that open the port now share
one failure path that names the group, and names the part that actually confuses
people: adding yourself with usermod -aG does NOT affect a shell that is already
running, because credentials are captured at login. So the same error greets
someone who has already fixed it. The message offers newgrp for this shell and sg
for one command.

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