No success or failure message when customizer settings are imported

[thebengalboy]

Description
There is no success or failure message displayed when we import a setting from a JSON file. The page simply reloads without doing anything else, yet the settings are successfully imported.

How to Reproduce
Please follow the steps:

• Install and activate the Import / Export Customizer Settings plugin
• Try importing a setting JOSN file.

ScreenCast
URL: https://d.pr/i/Qgrtzx

What it should be
It should show the success or failure message after the import action, maybe a sweet alert or something. And in case of failure, please add what was wrong exactly and what needs to be corrected before attempting to import again.