Sangaboard reports as /dev/ttyACM0

Currently, app.py line 74 tries to open a connection to the motor controller using /dev/ttyUSB0. On my microscope I don't have /dev/ttyUSB0, but I do have /dev/ttyACM0 which I think is equivalent. We should add fallback or autodetection code to cope with this.

Additionally, when I try to connect from eV all it says is "request failed with error 500". Particularly when connecting, it would be immensely helpful to pass along the actual error, because not everyone has a Joel on the end of reverse-SSH to analyse their log file...