Fix unhandledRejection on error inside function-based custom commands.

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 {
+                // most probably a dead code because all `instance` objects with `EventEmitter`
+                // superclass are expected to be function-style commands which will always have
+                // the `needsPromise` property set to `true`. But, not taking chances here.
+                instance.emit('error', err);
+              }
 
               return;
             }