Also adds an icon for error tiles.

Not satisfied with the current look of those markers, though.

At the moment,
Controller Character:
- Move: old marker
- Attack: old marker
- Can't Defend (Move): Diagonal dashes on the old marker
- Can't Defend (Attack): Diagonal dashes on the old marker
Targeted Character:
- Move: half-tile sized pawprint, centered.
- Attack: half-tile sized blade, centered.
- Can't Defend (Move): ???
- Can't Defend (Attack): ???

Also, clicking on the Targeted Character's markers should still be
considered as an input on the tile (or marker) below.