Propagation: Make 503 a fatal error.

Running further propagation jobs when the server is likely to be
503 for a time does not seem very useful.

src/libsync/owncloudpropagator_p.h

@@ -57,6 +57,12 @@ inline SyncFileItem::Status classifyError(QNetworkReply::NetworkError nerror, in
         return SyncFileItem::FatalError;
     }
 
+    if (httpCode == 503) {
+        // "Service unavailable"
+        // Happens for maintenance mode and other temporary outages
+        return SyncFileItem::FatalError;
+    }
+
     if (httpCode == 412) {
         // "Precondition Failed"
         // Happens when the e-tag has changed