processGraph(): Don't throw ThreadPoolShutDown if there is an exception

Fixes

  $ nix copy --derivation --to /tmp/nix /nix/store/...
  error: cannot enqueue a work item while the thread pool is shutting down

The ThreadPoolShutDown exception was hiding the reason for the thread
pool shut down, e.g.

  error: cannot add path '/nix/store/03sl46khd8gmjpsad7223m32ma965vy9-fix-static.patch' because it lacks a signature by a trusted key

src/libutil/thread-pool.hh

@@ -150,8 +150,16 @@ void processGraph(
         }
     };
 
-    for (auto & node : nodes)
+    for (auto & node : nodes) {
-        pool.enqueue(std::bind(worker, std::ref(node)));
+        try {
+            pool.enqueue(std::bind(worker, std::ref(node)));
+        } catch (ThreadPoolShutDown &) {
+            /* Stop if the thread pool is shutting down. It means a
+               previous work item threw an exception, so process()
+               below will rethrow it. */
+            break;
+        }
+    }
 
     pool.process();