Use promise then() instead of catch() with callbacks

Addresses my review here: https://github.com/webtorrent/webtorrent-desktop/pull/1419#pullrequestreview-285182318

src/renderer/lib/state.js

@@ -151,8 +151,10 @@ function setupStateSaved (cb) {
   })
 
   Promise.all(tasks)
-    .then(() => cb(null, saved))
-    .catch(err => cb(err))
+    .then(
+      () => cb(null, saved),
+      err => cb(err)
+    )
 
   function createTorrentObject (t) {
     // TODO: Doing several fs.readFileSync calls during first startup is not ideal

src/renderer/webtorrent.js

@@ -358,11 +358,15 @@ function getAudioMetadata (infoHash, index) {
     : mm.parseStream(file.createReadStream(), file.name, options)
 
   onMetaData
-    .then(() => {
-      console.log(`metadata for file='${file.name}' completed.`)
-    }).catch(function (err) {
-      return console.log('error getting audio metadata for ' + infoHash + ':' + index, err)
-    })
+    .then(
+      () => console.log(`metadata for file='${file.name}' completed.`),
+      err => {
+        console.log(
+          `error getting audio metadata for ${infoHash}:${index}`,
+          err
+        )
+      }
+    )
 }
 
 function selectFiles (torrentOrInfoHash, selections) {