Update.app need to catch "Process exit code: 1" better

Update.app when running on a read-only system.

We get "Process exit code: 1" but we don't get an error message on the screen and the indefinite status bar keeps running forever - this should be changed.

```
FreeBSD% cat /var/log/update.log
11-05 22:32 root INFO None
11-05 22:32 root INFO Proceeding to update all packages
11-05 22:32 root INFO Found QTextEdit for the details
11-05 22:32 root INFO Process started
11-05 22:32 root INFO sudo
11-05 22:32 root INFO ['-A', '-E', 'freebsd-update', 'fetch', 'install', '--not-running-from-cron']
11-05 22:32 root INFO src component not installed, skipped
11-05 22:32 root INFO freebsd-update: Directory is not on a persistent filesystem: /var/db/freebsd-update
11-05 22:32 root INFO Process finished
11-05 22:32 root INFO Process exit code: 1
```