feat(ui): support an error of "false" that exits without logging

- this can simplify some of the weird promise chaining in that if a
command needs to exit without doing anything else it can just reject
with false as the rejection

lib/ui.js

@@ -162,7 +162,8 @@ class UI {
         } else if (isObject(error)) {
             // TODO: find better way to handle object errors?
             this.log(JSON.stringify(error));
-        } else {
+        } else if (error !== false) {
+            // If the error is false, we're just exiting (makes the promise chains easier)
             this.log('An unknown error occured.');
         }
     }