bug: fix missing error message if a label can't be deleted

[JannikStreek]

Further Notes

If a label is used by an idea, it can't be deleted (that is correct). But an error should appear, which doesn't work right now.

The error is available in the f_label variable inside the .form_for component but it is currently not shown.

Old code: https://github.com/b310-digital/mindwendel/blob/88f7f2d56fce903525d40f82ba262ac3f0d114ab/lib/mindwendel_web/live/admin/brainstorming_live/edit.html.heex#L158C15-L168C24

Please also see the commented test: test/mindwendel_web/live/admin/brainstorming_live/edit_test.exs:125

Update: I tried to display the error with:

  <.error :for={msg <- Enum.map(f_label[:idea_idea_labels].errors, &translate_error(&1))}>
    <%= msg %>
  </.error>

The error then appears in html. However, it is assigned the invalid-feedback class and therefore hidden.

See also phoenixframework/phoenix_live_view@ed9268a

Possible related to #362