libhackrf hangs if HackRF is disconnected

[martinling]

Steps to reproduce

1. Start hackrf_transfer -r /dev/null
2. Unplug HackRF

Expected behaviour

Program exits with an appropriate error.

Actual behaviour

Program hangs, and can only be killed with SIGKILL.

Background

I decided to test this after writing this comment, because disconnection was the obvious case for when we would need to handle errors from libusb.

It looks like I broke this in #1029.

Running with gdb shows that the hang happens when hackrf_stop_rx() calls cancel_transfers(), which calls pthread_cond_wait to wait for cancellations to complete. But the transfer thread is already dead, because hackrf_libusb_transfer_callback saw an error and called request_exit. So the condition is never set.

We need to rethink the error handling path to work with the other changes. I'll work on that.