[laws] fix death message on unknown fiberSource

This was never reported by the compiler because DIE_MSG treated
its argument as a string. With the new stream formatting support the
compiler sees the undefined symbol flag.

emper/strategies/laws/LawsDispatcher.cpp

@@ -48,7 +48,7 @@ void LawsDispatcher::dispatchLoop() {
 						lawsStrategy.dispatchedFiberFromAnywhere.fetch_add(1, std::memory_order_relaxed);
 						break;
 					default:
-						DIE_MSG("Unknown fiber flag: " << flag);
+						DIE_MSG("Unknown fiber source: " << next.metadata);
 						break;
 				}
 			}