Missing docs on error handling

[dalechyn]

Seems like people don't know yet that frog can return error responses in frame, cast action and transaction handlers!

Ref: https://warpcast.com/justin-eth/0x86e7a5be, https://warpcast.com/samuellhuber/0x61c858cf

The actual docs that we have rn: https://frog.fm/reference/frog-transaction-context#error, https://frog.fm/reference/frog-frame-context#error, https://frog.fm/reference/frog-cast-action-context#error.

The goal would be to have a separate page that would explain how to return an error. This might also go into a "Best Practices" page.