Make firmware writing more robust

[Mrnt]

Please can you test with some different boards and scenarios. I only have access to a PowerSpec (which no longer completes the firmware update process normally*) and a Dreamer motherboard, but it seems to work OK with them.

My goal was to make the process of uploading the firmware a bit more robust. To that end, the changes do the following:

• check for multiple USB vendor ID's
• make initiating the control connection more robust
• display response from printer (useful for debugging a failure)
• handle USB exceptions more gracefully

*The PowerSpec now requires the SD card be removed after the firmware update or it gets stuck updating the firmware after every boot. Manually removing the power.bin file from the SD card resolves this problem.

[moonglow]

Thank you ! I will test it soon, my Python skills is absolutely zero, but i like what you make it more structable and readable for others !