jenkins: improve logging related to creating comments on GitHub

As the previous logging didn't provide enough details about what went
wrong; was it fetching info about existing comments, while editing
or creating comment?

lib/github-comment.js

@@ -46,16 +46,22 @@ exports.getFirstBotComment = function getFirstBotComment ({ owner, repo, number
 }
 
 exports.createPrComment = function createPrComment ({ owner, repo, number, logger }, body) {
-  exports.getFirstBotComment({ owner, repo, number, logger }).then((comment) => {
-    if (comment) {
-      const { id: nodeId, body: oldBody } = comment
+  exports.getFirstBotComment({ owner, repo, number, logger }).then((existingComment) => {
+    if (existingComment) {
+      const { id: nodeId, body: oldBody } = existingComment
       const newBody = `${oldBody}\n${body}`
       const id = graphQlIdToRestId(nodeId)
-      return githubClient.issues.editComment({ owner, repo, id, body: newBody })
+      const updatedComment = { owner, repo, id, body: newBody }
+
+      return githubClient.issues.editComment(updatedComment).catch((err) => {
+        logger.error({ existingComment, updatedComment, err }, 'Error while editing existing comment on GitHub')
+      })
     }
-    return githubClient.issues.createComment({ owner, repo, number, body })
-  }).catch((err) => {
-    logger.error(err, 'Error while creating comment on GitHub')
-    // swallow error
+
+    return githubClient.issues.createComment({ owner, repo, number, body }).catch((err) => {
+      logger.error(err, 'Error while creating comment on GitHub')
+    })
+  }, (err) => {
+    logger.error(err, 'Error while trying to fetch existing bot comment on GitHub')
   })
 }