Check for errors in spawned processes

[franglais125]

Currently if a check for updates or upgrade process fails, the extension:

• updates the last check time anyway
• does not give any feedback related to the problem

Instead, the exit status of the spawned program should be checked to ensure it succeeded.

Thanks @lucatrv !

[lucatrv]

Maybe it would be useful to add an option like "Try again if unsuccessful / Interval before attempting (minutes, 0 to disable)", with default "60".