jenkins: check successful response before reading (#247)

While having a look at recent error logs, this fatal log stood out:

```
02:48:11.721 FATAL bot: Unchaught exception, terminating bot process immediately
    TypeError: Cannot read property 'data' of undefined
        at githubClient.pullRequests.getCommits (/.../github-bot/lib/push-jenkins-update.js:92:29)
        at module.exports.sendError (/.../github-bot/node_modules/github/lib/index.js:874:7)
        at /.../github-bot/node_modules/github/lib/index.js:882:21
        at callCallback (/.../github-bot/node_modules/github/lib/index.js:738:9)
        at IncomingMessage.<anonymous> (/.../github-bot/node_modules/github/lib/index.js:807:13)
        at IncomingMessage.emit (events.js:194:15)
        at endReadableNT (_stream_readable.js:1125:12)
        at process._tickCallback (internal/process/next_tick.js:63:19)
```

Since we never want these kinds of errors to happen since they
end up killing the bot process, this fix is pushed to ensure we
don't naively read the response from github.com until we know the
request didn't fail for some reason.

Refs #246

lib/push-jenkins-update.js

@@ -89,11 +89,11 @@ function findLatestCommitInPr (options, cb, pageNumber = 1) {
     page: pageNumber,
     per_page: 100
   }, (err, res) => {
-    const commitMetas = res.data || []
     if (err) {
       return cb(err)
     }
 
+    const commitMetas = res.data || []
     const lastPageURL = githubClient.hasLastPage(res)
     if (lastPageURL) {
       return findLatestCommitInPr(options, cb, pageNumberFromURL(lastPageURL))