Make Error comparable

[bytedream]

Description

This PR makes Error comparable. Useful in situations where a custom error enum is used.

This is useful in situation where, for example, a custom error enum is used:

#[derive(Eq, PartialEq, thiserror::Error)]
enum CustomError {
    #[error("error: {0}")]
    Normal(String),
    #[error("influx error: {0}")]
    Influx(#[source] influxdb::Error)
}

#[cfg(test)]
mod tests {
  use super::*;

  fn test() {
      let err = CustomError::Normal("err".to_string());
      let err1 = CustomError::Normal("err".to_string());
  
      // this assert only works when `PartialEq` is implemented/derived for `CustomError`
      assert_eq!(err, err1)
  }
}

Checklist

• Formatted code using cargo fmt --all
• Linted code using clippy (there are warnings, but the reasons for them are not my additions)
  • with reqwest feature: cargo clippy --manifest-path influxdb/Cargo.toml --all-targets --no-default-features --features use-serde,derive,reqwest-client -- -D warnings
  • with surf feature: cargo clippy --manifest-path influxdb/Cargo.toml --all-targets --no-default-features --features use-serde,derive,hyper-client -- -D warnings
• Updated README.md using cargo doc2readme -p influxdb --expand-macros
• Reviewed the diff. Did you leave any print statements or unnecessary comments?
• Any unfinished work that warrants a separate issue captured in an issue with a TODO code comment

[msrd0]

Hi, thanks for your PR. Why do you compare the Display version of the errors, instead of using the derive macro for (Partial)Eq?

[bytedream]

Good point lol. I've just took over the Eq part of my implementation before this PR. Adjusted it

[Empty2k12]

Thanks! 🚀