rest-server: worker task: include context in state error message

Currently the anyhow error context of a given error is not included
in the error message, as `to_string` does use the default formatting
[0].

Include the error context, formatting it as single line as the
message is also shown to the users in e.g. the Proxmox Backup Severs
task state in the UI.

[0] https://docs.rs/anyhow/latest/anyhow/struct.Error.html#display-representations

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

proxmox-rest-server/src/worker_task.rs

@@ -1003,7 +1003,7 @@ impl WorkerTask {
 
         if let Err(err) = result {
             TaskState::Error {
-                message: err.to_string(),
+                message: format!("{err:#}"),
                 endtime,
             }
         } else if warn_count > 0 {