better handling of disconnect during flash

[dlech]

Currently the behavior (error message) is different on each OS when a hub is disconnected while flashing. Linux works as expected. Windows will say that the connection timed out. Mac will say that there was an unknown GATT error.

Ideally all OSes should give the correct error that the hub was disconnected.

To reproduce, start flashing and pull the battery when the spinner in the toolbar button start spinning.