Fix unhandledRejection on error inside function-based custom comman…

…ds. (nightwatchjs#4309)

lib/api/_loaders/command.js

@@ -183,7 +183,16 @@ class CommandLoader extends BaseCommandLoader {
           })
           .catch(err => {
             if (instance instanceof EventEmitter) {
-              instance.emit('error', err);
+              if (instance.needsPromise) {
+                // if the instance has `needsPromise` set to `true`, the `error` event is listened
+                // on the `context` object, not on the `instance` object (in `treenode.js`).
+                this.emit('error', err);
+              } else {
+                // for class-based commands that inherit from EventEmitter.
+                // Since the `needsPromise` is set to `false` in this case, the `complete` and `error`
+                // events are listened on the `instance` object.
+                instance.emit('error', err);
+              }
 
               return;
             }

lib/core/treenode.js

@@ -206,6 +206,9 @@ class TreeNode {
       commandResult = this.instance;
       if (this.instance.needsPromise) {
         this.needsPromise = true;
+        // this change was done because the only way function-styled custom commands could be resolved
+        // is if they contain another NW API command, which could call `node.context.emit('complete')`
+        // inside `asynctree.js > resolveNode` method for the custom command node.
         commandResult = this.context;
       }
     } else if (this.context instanceof EventEmitter) { // Chai assertions