Missing -p cmd line

[ril3y]

If you do not provide a -p the programs crashes with non helpful message.
@giseburt

[giseburt]

This is related to #6 and a similar issue (not actually recorded in github issues yet) on Linux, where if the auto-detect doesn’t find a TinyG or g2core machine it fails.

The error message is getting lost due to another bug with error handling, resulting in a non-helpful exit message.