fix panic when upstream fails (#412)

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/412
Co-authored-by: crapStone <me@crapstone.dev>
Co-committed-by: crapStone <me@crapstone.dev>

server/upstream/upstream.go

@@ -183,9 +183,12 @@ func (o *Options) Upstream(ctx *context.Context, giteaClient *gitea.Client, redi
 			// add extension for encoding
 			path := o.TargetPath + allowedEncodings[encoding]
 			reader, header, statusCode, err = giteaClient.ServeRawContent(o.TargetOwner, o.TargetRepo, o.TargetBranch, path, true)
-			if statusCode == 404 {
+			if statusCode == http.StatusNotFound {
 				continue
 			}
+			if err != nil {
+				break
+			}
 			log.Debug().Msgf("using %s encoding", encoding)
 			if encoding != "identity" {
 				header.Set(headerContentEncoding, encoding)