Fix: Show error message if user settings can't be saved

When the http request returned an error the error message was not shown
while changing the user settings in the corresponding dialog. It broke
while introducing the JS form validation because its validate function
isn't a promise in contrast to the onSave handler.

src/web/pages/usersettings/dialog.js

@@ -15,7 +15,7 @@
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
-import React, {useState} from 'react';
+import React, {useCallback, useState} from 'react';
 
 import styled from 'styled-components';
 
@@ -169,11 +169,17 @@ let UserSettingsDialog = ({
   const [error, setError] = useState();
   const [formValues, handleValueChange] = useFormValues(settings);
 
+  const handleSave = useCallback(values => {
+    onSave(values).catch(err => {
+      setError(err.message);
+    })
+  }, [onSave]);
+
   const {hasError, errors, validate} = useFormValidation(
     userSettingsRules,
     formValues,
     {
-      onValidationSuccess: onSave,
+      onValidationSuccess: handleSave,
       onValidationError: setError,
       fieldsToValidate,
     },