Show error when attempting to change your username to one that's already taken

[tjb]

Describe the bug
When I attempt to change my username I am not able to. There is no reason displayed to show why my change was not accepted.

To Reproduce
Steps to reproduce the behavior:
Navigate to the Settings page. From here navigate to My Account. Click on the username and change the username. Click Save. The new username does not save and the console outputs a the following:

The operation couldn’t be completed. (Server error 400.)

Expected behavior
The expected behavior would be a toast that would show a success or error of the attempted save.

Smartphone (please complete the following information):

• Device: iPhone 14 Pro (Simulator)
• OS: iOS 16.4
• App Version: 3.7.1 (856) simulator
• Habitica Username: tyfigher

Additional context
I tried the same username change on the web application and it shows me that the username I am attempting to change to is already taken. The toast should give some detail as to why I cannot save.