Check API response for failure

Fix #30

Author: zachwhaley

content.js

@@ -90,6 +90,10 @@ async function getPrMetadataFromDocument() {
 
 async function getPrMetadataFromApi(repo, prNumber) {
   const response = await fetch(`https://api.github.com/repos/${repo}/pulls/${prNumber}`);
+  if (!response.ok) {
+    return null;
+  }
+
   const prMetadata = await response.json();
   return {
     // These could be null in some cases, so replace nulls with empty strings.