Expose files backup errors to the user

[t-m-w]

Come up with a way to expose errors to the user as it relates to Storage backup.

Currently, errors backing up files are ignored in the hope that they will be resolved in the next backup run. However, in some circumstances, this won't happen - for example, if the backup destination is rejecting our uploads due to a client body size greater than it supports, as is the case with nginx's default of 1 MiB (when not overridden) and the WebDAV storage option.

[grote]

We are now showing the number of files that could not be restored due to errors. But I guess this is about backup.

Related to #727