optionally implement defmt::export::fmt behind a defmt flag for bleps::Error: Format

[brandonros]

It would fix this error:

the trait bound `bleps::Error: Format` is not satisfied
the following other types implement trait `Format`:
  &T
  &mut T
  ()
  (T0, T1)
  (T0, T1, T2)
  (T0, T1, T2, T3)
  (T0, T1, T2, T3, T4)
  (T0, T1, T2, T3, T4, T5)
and 191 others

from this code:

    defmt::unwrap!(ble.init().await);