Add an error message service that shows a popup error

This can be used to validate for example: ship elements such as guns are only placed on top of hulls. Although then when that hull is deleted with eraser, it will need to somehow delete the image on top too.

Start by locating the editor's validation and eraser entry points, then trace how ship elements such as guns and hulls are handled. Define the popup error behavior for invalid placement and the cleanup behavior when a hull is erased. No file or test is named, so the implementation scope needs clarification before work begins.

Written by the indexing model from the issue text.