Switch to let _ = to ignore errors and explain why

Author: carols10cents

src/krate.rs

@@ -828,9 +828,12 @@ pub fn download(req: &mut Request) -> CargoResult<Response> {
     let crate_name = &req.params()["crate_id"];
     let version = &req.params()["version"];
 
-    match increment_download_counts(req, crate_name, version) {
-        _ => {} // do not try, ignore failures
-    }
+    // Ignore failure to update download counts. We might be a mirror that
+    // doesn't have the crates available in the database (so can't look them
+    // up) and therefore doesn't care about the download counts, but we can
+    // still construct valid crate URLs. Cargo will verify that the crate and
+    // version are in the crate index before making a request to this route.
+    let _ = increment_download_counts(req, crate_name, version);
 
     let redirect_url = format!("https://{}/crates/{}/{}-{}.crate",
                                req.app().bucket.host(),