HardwareManager hangs when bled112 is unplugged

[JKHeadley]

ref: https://github.com/iotile/iotile-factory-companion/pull/209#issuecomment-659042377

steps to reproduce: 1) run iotile hw --port=bled112 with bled112 plugged in 2) unplug bled112 3) run connect. (Ex: connect 0x0a49)

• should see several errors appear
• note that the (HardwareManager) prompt does not reappear (i.e. the previous command hangs)
• ctrl-c does show the (HardwareManager) prompt, however subsequent commands don't work correctly

Note that the same behavior occurs if you swap steps 2 and 3 and run a command afterwards.

[JKHeadley]

@timburke fyi