Sync: re-factor error handling

[rfc2822]

Currently, handling of sync errors is scattered between various locations:

• Sync worker throws and handles some errors and show notifications for some other errors
• Syncer throws errors (for instance when the content provider is not available, after #877) and handles others (SecurityException, DeadObjectException)
• SyncManager throws and handles other errors (per collection)

We should

• [ ] clearly define which component is responsible for throwing which error,
• [ ] define which component is responsible for handling which error in which way (notification, instant/with retries, …)
• [ ] show a notification when the content provider can't be acquired at sync during to missing permissions or other reasons (see https://github.com/bitfireAT/davx5-ose/pull/877#pullrequestreview-2159419428)

and refactor accordingly.

Maybe have a separate (injectable) error manager that does notifications and for instance can also group errors of various collections? But that could also be a new issue.

Depends on #877. See also #886 – it probably makes sense to think about error handling when defining the sync result data type.

Depends on #896

[github-actions[bot]]

This PR/issue depends on:

• bitfireAT/davx5-ose#877
• bitfireAT/davx5-ose#896