Right now it is unclear what kind of errors a contract (or even a function) can fail with. It would be nice to have a standardized way of declaring contract errors, something like this:

#[near_error]
pub enum MyError {
  #[message("Caller is not the owner of this contract")]
  NotTheOwner,
  #[message("Caller is not allowed to call this function")]
  NotAllowed,
}

We can then include all contract errors as a part of the ABI, this proposal also goes really well with the Result-based error handling as we can statically infer what errors a function can fail with.