enhancement: give the user some hint as to why an export failed

[svenb1234]

Some attempt to export failed. I got that message via the client and on the web interface. "Export of x failed."

This does not give any hint to the user what the reason for failing was. Hence the user cannot change anything in order to make the export work.

A normal user cannot check the logs of the nextcloud installation and is thus stuck.

[Pytal]

Most failures would probably require a sysadmin to resolve so we could add a message to contact your system administrator @come-nc?

[svenb1234]

So the app does not know whether it failed at gathering the DB, or which folder it was packing, which file etc.?

[Pytal]

The total number of ways that an export could fail is too large to handle with such granularity, not sure if it would be worth it @come-nc?

[come-nc]

Not sure it is easy to give details to the user without security issues. We cannot leak to the user the data path, or the database used and so on.

I will not work on that I am open to reviewing PR giving more information about failures, as long as the concern I expressed above is adressed.

[Pytal]

Feel free to open a PR @svenb1234 :)