Error on splash screen if sync is failing

Display an error on the splash screen with the spinner if the sync
request is not working, rather than just sitting there with a spinner
as if nothing is wrong.

Fixes https://github.com/vector-im/riot-web/issues/7148

res/css/structures/_MatrixChat.scss

@@ -56,6 +56,18 @@ limitations under the License.
     flex: 1;
 }
 
+.mx_MatrixChat_syncError {
+    color: $accent-fg-color;
+    background-color: $warning-bg-color;
+    border-radius: 5px;
+    display: table;
+    padding: 30px;
+    position: absolute;
+    top: 100px;
+    left: 50%;
+    transform: translateX(-50%);
+}
+
 .mx_MatrixChat .mx_LeftPanel {
     order: 1;