storcon: return an error for drain attempts while paused (#9997)

## Problem

We currently allow drain operations to proceed while the node policy is
paused.

## Summary of changes

Return a precondition failed error in such cases. The orchestrator is
updated in neondatabase/infra#2544 to skip drain
and fills if the pageserver is paused.

Closes: #9907

storage_controller/src/service.rs

@@ -5681,7 +5681,7 @@ impl Service {
         }
 
         match node_policy {
-            NodeSchedulingPolicy::Active | NodeSchedulingPolicy::Pause => {
+            NodeSchedulingPolicy::Active => {
                 self.node_configure(node_id, None, Some(NodeSchedulingPolicy::Draining))
                     .await?;