Fix DoubleRenderError in update action and simplify display_errors

[Solus90]

What

This PR fixes two bugs in RestroomsController:

Bug 1: AbstractController::DoubleRenderError in update

In the update action, when a restroom fails validation during an edit, the code called both render 'edit' and then fell through to the unconditional redirect_to @restroom below it. This raises an AbstractController::DoubleRenderError at runtime.

Before:

else
  display_errors
  render 'edit'
end

redirect_to @restroom  # always executes — crashes when render already called

After:

else
  display_errors
  render 'edit'
  return  # prevents fall-through to redirect_to
end

redirect_to @restroom

Bug 2: Redundant iteration in display_errors

The display_errors helper iterated over @restroom.errors.each do but had no block parameter. On every iteration it simply overwrote the same flash[:alert] key with the same value — a no-op loop that just sets the same string repeatedly and then discards it.

Before:

@restroom.errors.each do
  flash[:alert] = I18n.t('restroom.flash.field')
end

After:

flash[:alert] = I18n.t('restroom.flash.field')

Impact

• Editing an existing restroom and submitting invalid data would previously crash with a DoubleRenderError instead of showing the edit form with an error message.
• No behaviour change for valid submissions or voting actions.