Bugfix: Don't abort the find command when a tree can't be loaded

Change the find command so that missing trees don't result in a crash.
Instead, the error is logged to the debug log, and the tree ID is displayed
along with the snapshot it belongs to.  This makes it possible to recover
repositories that are missing trees by forgetting the snapshots they are used
in.

https://github.com/restic/restic/issues/2224
