.moves() can provide moves that throw an "invalid move" error

[StevenStavrakis]

When moving a pawn to a promotion square, if the move is made without including a promotion option, the move will throw an error. However, .moves() shows it as a legal move.

Recommended solution
A different error should be thrown if a pawn is moved to a promotion square without providing a promotion argument. That way, the error can be caught and handled in the UI.