Fix wall preview to show unaffordable segments in red

[historic_bruno]

This bug has annoyed me a few times. It appears that I can build a long wall when the preview looks normal, if I don't pay attention to the exact cost and my current resource count, it will only build part of the wall which can be totally useless especially given #1566. To make this clearer, it should show unaffordable parts of the wall in red, which may be the entire wall or only the end of it (I don't think it's bad to reuse the red shading from build restrictions in this case).

[Deiz]

In 12569:

Shade and show an insufficient resources tooltip for unaffordable walls. Fixes #1641.