WIP: _sandboxbwrap.py: Catch ENOTCONN when umounting

Description

[//]: # Remove backtrace when umounting devices which are not present or not able to connect

[//]: # Wait until the device is connected, this seems to happen in a timeout bigger than 30 seconds

Changes proposed in this merge request:

• Catch ENOTCONN when umount device
• Continue with the umount operation until succeed (testing more than 3 times with not issues)

[//]: # Bug #161

This merge request, when approved, will close issue/bug: #161

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