message: Fixes a crash in spec translator.
  Crash happened when operations are applied to helper functions for which we don't know the result type.
  Closes:
  Approved by: DavidLDill
diff: @@ -340,13 +340,23 @@ impl<'env> SpecTranslator<'env> {
  format!("i#Integer({}) {} i#Integer({})", l, op, r)
  }
  GlobalType::Bool => format!("b#Boolean({}) {} b#Boolean({})", l, op, r),
  - _ => panic!("unexpected type"),
  + &UNKNOWN_TYPE => self.error(
  + "unknown result type of helper function; cannot use in operation",
  + "<error>".to_string(),
  + ),
  + &ERROR_TYPE => "<error>".to_string(),
  + _ => panic!("unexpected type: {:?}", expected_operand_type),
  };
  match result_type {
  GlobalType::U8 | GlobalType::U64 | GlobalType::U128 => {
  BoogieExpr(format!("Integer({})", expr), result_type)
  }
  GlobalType::Bool => BoogieExpr(format!("Boolean({})", expr), result_type),
  + UNKNOWN_TYPE => self.error(
  + "unknown result type of helper function; cannot use in operation",
  + BoogieExpr("<error>".to_string(), ERROR_TYPE),
  + ),
  + ERROR_TYPE => BoogieExpr("<error>".to_string(), ERROR_TYPE),
  _ => panic!("unexpected type"),
  }
  }
  