Fix bwc types

Author: nicktindall

server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -192,7 +192,7 @@ public static ThreadPoolType bwcType(String threadPoolName) {
                 case Names.GENERIC:
                     return SCALING;
                 default:
-                    throw new IllegalArgumentException("no bwc type for " + threadPoolName);
+                    return THREAD_POOL_TYPES.get(threadPoolName);
             }
         }
     }