improve error handling

[DeeNewcum]

This adds two new errors:

• "Error: ./pi400 must be run as root"
• "Error: This must be added to /boot/config.txt and the system rebooted: dtoverlay=dwc2"

Without this error-checking, the beginner user gets some strange errors that don't really correspond to what they need to fix to proceed.

[DeeNewcum]

I'm a little confused what #define NO_OUTPUT is for, or whether the call to prechecks() should be wrapped in it.

[Gadgetoid]

NO_OUTPUT is a slightly hamfisted way to disable output to the USB gadget, so the code can be tested on a system without it supported or set up.

IIRC you need root either way, for grabbing the local devices but I could be misremembering.

But yes, the dtoverlay=dwc2 check should be wrapped in #IFNDEF NO_OUTPUT

[Gadgetoid]

Thank you. I'll try to get this tested & merged this week.