Enable best-effort backups

[ashmrtn]

Best-effort backups encapsulate the idea that as long as some data for an operation was backed up, the data that was successfully backed up should be available for future restores. This data can also play a (maybe minimal) role in some optimizations like kopia-assisted incrementals

However, enabling this in a safe manner is complex as it touches many different areas of the system and requires careful aggregation of errors to ensure everything is properly accounted for

Things that need to be addressed in order to enable this are:

• CLI output for backups where one or more items could not be retrieved
• setting the BackupOp status
• excluding backups with some failed items from delta-token based incrementals #2208
• allowing backups with some failed items for kopia-assisted incrementals #2209