download: check for failure status codes

Check for HTTP status codes for immediate failures in download_and_hash,
for now only 403 Forbidden and 404 Not found.

src/download.rs

@@ -2,6 +2,8 @@ use std::error::Error;
 use std::io::Write;
 use std::io;
 
+use reqwest::StatusCode;
+
 use sha2::{Sha256, Digest};
 
 pub struct DownloadResult<W: std::io::Write> {
@@ -14,11 +16,23 @@ where
     U: reqwest::IntoUrl,
     W: io::Write,
 {
+    // All HTTP status codes that must result in an immediate failure.
+    let failure_status = [
+        StatusCode::FORBIDDEN, // 403
+        StatusCode::NOT_FOUND, // 404
+    ];
+
     #[rustfmt::skip]
     let mut res = client.get(url)
         .send()
         .await?;
 
+    // Return immediately on download failure on the client side.
+    let status = res.status();
+    if status.is_client_error() && failure_status.contains(&status) {
+        return Err(format!("cannnot fetch remotely with status code {:?}", status).into());
+    }
+
     let mut hasher = Sha256::new();
 
     let mut bytes_read = 0usize;