output/alsa: rethrow errors in Drain()

src/output/plugins/AlsaOutputPlugin.cxx

@@ -762,13 +762,20 @@ AlsaOutput::Drain()
 {
 	const std::lock_guard<Mutex> lock(mutex);
 
+	if (error)
+		std::rethrow_exception(error);
+
 	drain = true;
 
 	UnlockActivate();
 
-	while (drain && !error)
+	while (drain) {
+		if (error)
+			std::rethrow_exception(error);
+
 		cond.wait(mutex);
 	}
+}
 
 inline void
 AlsaOutput::CancelInternal() noexcept