Review system update code and check error handling

[lucarin91]

There are multiple complaints about the update issue; probably most/some of them are spurious failures, so we need to check that errors are meaningful and it is clear how and if we can retry.

We should do some testing in different conditions:

• with app lab (via api)
• with the command line
• simulate a poor internet connection
• after the clean re-flash of the board

Another issue that maybe could be confirmed and reproduced in this review is that in some situations in which the update goes in the background, but the events endpoint doesn't return the logs.

Additional context

Related

• return errors of system update commands #8
• system update doesn't print/log errors #16
• The update fails at the first boot of the board #53