refactor: avoid non_fmt_panics warning

[kamadorueda]

• Avoid the following warning:

  warning: panic message is not a string literal
  --> src/rules.rs:815:29
    |
  815 |         .map_err(|e| panic!(e))
    |                             ^
    |
    = note: `#[warn(non_fmt_panics)]` on by default
    = note: this usage of panic!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
  help: add a "{}" format string to Display the message
    |
  815 |         .map_err(|e| panic!("{}", e))
    |                             +++++

[zimbatm]

thanks!