enterDfuMode should catch UsbError

[nrobinson2000]

https://community.particle.io/t/connection-being-lost-when-flashing-via-usb/59315/3?u=nrobinson2000

In the linked thread I discuss the findings of my research into a bug a user on the forum encounters when attempting to flash firmware in Workbench on Mac.

Overall, I think the catch in enterDfuMode should return when a UsbError is caught, as this is indicative that a device has (likely) entered DFU mode, but an error has occurred in the process. Catching the UsbError would eliminate the annoying one-minute timeout the user experiences.

[nrobinson2000]

@busticated Have you encountered this before?