fix: fetch_url: return err on non 2xx reponses

The main reason for this change is the app picker
that Delta Chat clients use, which utilizes
the `fetch_url` function.
Sometimes we get an error from the server,
but we have no way to figure out that it's an error,
other than inspecting the body, which we don't (and shouldn't) do.
This results in us attempting to send webxdc apps
that are not even valid .zip files.

Another, arguably even worse thing is that
we also put the error responses to the cache,
so it's not easy to recover from such an error.

So, let's just return an error if the response code
is not a successful response code.

src/net/http.rs

@@ -261,6 +261,18 @@ async fn fetch_url(context: &Context, original_url: &str) -> Result<Response> {
             continue;
         }
 
+        if !response.status().is_success() {
+            return Err(anyhow!(
+                "The server returned a non-successful response code: {}{}",
+                response.status().as_u16(),
+                response
+                    .status()
+                    .canonical_reason()
+                    .map(|s| format!(" {s}"))
+                    .unwrap_or("".to_string())
+            ));
+        }
+
         let content_type = response
             .headers()
             .get("content-type")