Don't duplicate error messages

[konstin]

When showing the error chain, reqwest duplicates all the error messages:

For a timeout:

💥 my-app failed
  Caused by: Network request failed
  Caused by: error sending request for url (https://httpbin.org/): operation timed out
  Caused by: operation timed out

For no internet connection:

💥 my-app failed
  Caused by: Network request failed
  Caused by: error sending request for url (https://httpbin.org/): error trying to connect: dns error: failed to lookup address information: Temporary failure in name resolution
  Caused by: error trying to connect: dns error: failed to lookup address information: Temporary failure in name resolution
  Caused by: dns error: failed to lookup address information: Temporary failure in name resolution
  Caused by: failed to lookup address information: Temporary failure in name resolution

The inlining that causes the duplication happens here:

reqwest/src/error.rs

Lines 209 to 211 in 68547a0

	if let Some(e) = &self.inner.source {
	write!(f, ": {e}")?;
	}

This is unlike what most of our other dependencies do and makes error quite verbose. Would it be possible to not inline the source field and leave that to the user?

For reproduction, the example program:

use std::time::Duration;
use reqwest::ClientBuilder;
use thiserror::Error;

#[derive(Debug, Error)]
enum MyError {
    #[error("Network request failed")]
    NetworkError(#[from] reqwest::Error)
}

async fn run() -> Result<(), MyError> {
    let client = ClientBuilder::new().timeout(Duration::from_millis(1)).build()?;
    client.get("https://httpbin.org").send().await?.text().await?;
    Ok(())
}

#[tokio::main]
async fn main() {
    if let Err(err) = run().await {
        eprintln!("💥 my-app failed");

        let mut last_error: Option<&(dyn std::error::Error + 'static)> = Some(&err);
        while let Some(err) = last_error {
            eprintln!("  Caused by: {err}");
            last_error = err.source();
        }
    }
}

reqwest = {  git = "https://github.com/seanmonstar/reqwest", rev = "68547a0af66222960eea6d649bf94497c11df246" }
thiserror = "1.0.57"
tokio = { version = "1.36.0", features = ["macros", "rt-multi-thread"] }