The most common reason for pySim to crash is when it is executed without
commandline parameters. Then pySim will expect a serial reader on
/dev/ttyUSB0 since this is the default. Lets check if /dev/ttyUSB0 even
exists before trying to open it.
Change-Id: I7545c728b531e9a796eee8f80f0b08d4097f8399