message: Make SystemError non_exhaustive
diff: @@ -14,6 +14,7 @@ use crate::HumanAddr;
  /// the contract understands the error format without creating a dependency on cosmwasm-vm.
  #[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
  #[serde(rename_all = "snake_case")]
  +#[non_exhaustive]
  pub enum SystemError {
  InvalidRequest { error: String },
  NoSuchContract { addr: HumanAddr },
  