libstore/http-binary-cache-store: Improve error messages in HttpBinaryCacheStore::upsertFile

Now the error message doesn't cram everything into a single line and we now instead get:

error:
       … while uploading to HTTP binary cache at 's3://my-cache?endpoint=http://localhost:9000?compression%3Dzstd&region=eu-west-1'

       error: unable to download 'http://localhost:9000/my-cache/nar/1125zqba8cx8wbfa632vy458a3j3xja0qpcqafsfdildyl9dqa7x.nar.xz': Operation was aborted by an application callback (42)

Author: xokdvium

src/libstore/http-binary-cache-store.cc

@@ -157,7 +157,9 @@ void HttpBinaryCacheStore::upsertFile(
     try {
         getFileTransfer()->upload(req);
     } catch (FileTransferError & e) {
-        throw UploadToHTTP("while uploading to HTTP binary cache at '%s': %s", config->cacheUri.to_string(), e.msg());
+        UploadToHTTP err(e.message());
+        err.addTrace({}, "while uploading to HTTP binary cache at '%s'", config->cacheUri.to_string());
+        throw err;
     }
 }