message: improve error conversion test
diff: @@ -201,7 +201,7 @@ impl From<PurseLookupError> for Error {
  mod tests {
  use std::convert::TryFrom;
  - use super::{Error, MAX_ERROR_VALUE};
  + use super::{Error, TryFromU8ForError, MAX_ERROR_VALUE};
  #[test]
  fn error_round_trips() {
  @@ -212,8 +212,10 @@ mod tests {
  "value of variant {} ({}) exceeds MAX_ERROR_VALUE ({})",
  error, i, MAX_ERROR_VALUE
  ),
  - Err(_) if i > MAX_ERROR_VALUE => (),
  - Err(_) => panic!("missing conversion from u8 to error value: {}", i),
  + Err(TryFromU8ForError(())) if i > MAX_ERROR_VALUE => (),
  + Err(TryFromU8ForError(())) => {
  + panic!("missing conversion from u8 to error value: {}", i)
  + }
  }
  }
  }
  