Provide error message if manually triggered app update fails due to backup failure

[ChristopherMag]

This may be something that I just need to learn better but I tried to manually update apps over the last few days by clicking the update button for the individual app tiles and each time it would start and then look like it finished but no update took place.

I eventually figured out today that this was due to the mount point where backups are sent not being available and after fixing that updates complete as expected but before this point they were failing silently when manually triggered.

I am not sure where I would look to see the reasons the update failed as the app's logs don't show anything and the Cloudron event log shows the update being triggered but doesn't show anything indicating that the update failed and subsequently why it failed.

[robi]

Yes, more backup checks needed.

[p44]

@robi Yes, more checks to keep the mounting "alive".