Add second entity (a block)

Needed to modify user-initiated actions to always return all entities
along with the changed hero. Else, the second entity disappeared.
Add error handling to facilitate work at the REPL

On error, the screen will go blank. You can restart the game with:

    (in-ns 'our-first-game.core)
    (on-gl (set-screen! our-first-game main-screen))
Move block so hero stands on it
Allow move speed to be configurable