Add debugging message for #443

This commit doesn't fix #443, but closes it until we can reproduce again the error.

src/utils.js

@@ -78,6 +78,13 @@ const upload = async (opts) => {
   const response = await fetch(endpoint, { method: 'POST', headers, body });
   const uri = await response.text();
 
+  if (!uri) {
+    console.log(response);
+    throw new Error(
+      'Malformed response; please report at https://github.com/iterative/cml/issues/443'
+    );
+  }
+
   return { uri, mime, size };
 };