db-snapshot: Fix error reporting when using "garage meta snapshot --all"

Snapshot errors on remote nodes were not reported at all.

We now get proper error output such as:

    0fa0f35be69528ab  error: Internal error: DB error: LMDB: No space left on device (os error 28)
    88d92e2971d14bae  ok

Fix #920

src/garage/admin/mod.rs

@@ -484,7 +484,7 @@ impl AdminRpcHandler {
 							AdminRpc::MetaOperation(MetaOperation::Snapshot { all: false }),
 							PRIO_NORMAL,
 						)
-						.await
+						.await?
 				}))
 				.await;