proxy: fix error handling in prune scheduling

we rely on the jobstate handling to write the error of the worker
into its state file, but we used '?' here in a block which does not
return the error to the block, but to the function/closure instead

so if a prune job failed because of such an '?', we did not write
into the statefile and got a wrong state there

instead use our try_block! macro that wraps the code in a closure

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/bin/proxmox-backup-proxy.rs

@@ -434,7 +434,7 @@ async fn schedule_datastore_prune() {
 
                 job.start(&worker.upid().to_string())?;
 
-                let result = {
+                let result = try_block!({
 
                     worker.log(format!("Starting datastore prune on store \"{}\"", store));
                     worker.log(format!("task triggered by schedule '{}'", event_str));
@@ -463,7 +463,7 @@ async fn schedule_datastore_prune() {
                         }
                     }
                     Ok(())
-                };
+                });
 
                 let status = worker.create_state(&result);