Remove the error message for "This position is already taken"

[bryanbraun]

Currently, we display hover/focus states on locations with existing pieces, and then display an error when a person tries to place a piece there:

Instead, our hover/focus should make it clear that there's no interactivity to be expected when clicking on a location with an existing piece. If we do that well, then no error should be necessary. Clicking will just do nothing and that will be enough of an indication that you can't put a piece there.

This can be considered alongside https://github.com/bryanbraun/connect-four/issues/3 to see what the ideal set of combined hover/focus behaviors should look like.