LibUsb Get Device List - Error Code Handling

[DSheirer]

Describe the bug

On some Linux systems, the TunerManager attempts to discover USB devices. In doing so, it calls LibUsb.getDeviceList() which returns a list of available devices, or it returns a negative value indicating an error code. Currently, sdrtrunk is ignoring the error code and only handling the positive tuner count.

Update sdtrunk to translate the returned negative (ie error) number and consider additional troubleshooting steps that might help to identify what the error is.

Since the intial call to getDeviceList() produces an error, any subsequent call to LibUsb.freeDeviceList() results in an error because the device list was not initialized, so don't call free device list.

To Reproduce
Not reproducible. As reported by a user.

Screenshots

Desktop (optional - complete the following information):

• OS: Linux

Additional context
As reported on Discord by user @roosta

[DSheirer]

Consider closing #1600 as a duplicate to this issue.