WeDo 2.0 can "go away" without causing any error

[cwillisf]

If a WeDo 2.0 goes away unexpectedly, such as by holding the power button to turn it off, Scratch Link does not currently inform the client of an error or close the session. It should do both.

Related to #85

[evhan55]

I believe this is macOS only?
(From our internal test report, the person testing on Windows did not report this same bug)

[ericrosenbaum]

I noticed this again when testing the behavior of the extension when removing the battery from a WeDo. The extension does not show a disconnect alert. In one case when I did this, Scratch Link (on mac) froze- the cursor changed to a beach ball when mousing over it, and I had to quit and restart it to reconnect.