[Feature] improve error messages on sol invocations

[prestwich]

          Gotcha, makes sense. Maybe update the error message with a suggestion?

Also, I'm guessing if you wanted rpc calls you just do:

sol!(
  #[sol(rpc)]
  "../examples/erc20/src/test.sol"
);

Feel free to close 👍

Originally posted by @alexfertel in #636 (comment)