message: Make BackendError non_exhaustive
diff: @@ -158,6 +158,7 @@ pub trait Querier {
  pub type BackendResult<T> = (core::result::Result<T, BackendError>, GasInfo);
  #[derive(Error, Debug, PartialEq)]
  +#[non_exhaustive]
  pub enum BackendError {
  #[error("Panic in FFI call")]
  ForeignPanic {},
  