Prevent LIBUSB_TRANSFER_STALL

[positlabs]

I'm getting LIBUSB_TRANSFER_STALL when trying to set a few values.

• absoluteExposureTime
• absolutePanTilt
• whiteBalanceTemperature

Testing on Logitech C920 on OSX 10.12.5.

https://github.com/positlabs/obscura-webcam-controller

[positlabs]

I'm guessing it means these controls are not supported.

http://libusb.sourceforge.net/api-1.0/group__asyncio.html#gga9fcb2aa23d342060ebda1d0cf7478856a23cdfd929f5fe82cca9654f8075eeebb

Even though control endpoints do not halt, a completed control transfer may have a LIBUSB_TRANSFER_STALL status code. This indicates the control request was not supported.

[positlabs]

Implementing a solve for #22 would allow the dev (or this lib) to detect supported controls, and only surface those - or throw a more descriptive error if a user tries accessing an unsupported control

[positlabs]

I'm going to add an initialization step in #22 so the user shouldn't ever access unsupported controls