Improve Error Handling for Duplicate User Registration

[tristenwallace]

Description:

Currently, when a user attempts to register with a username that is already taken, the server returns a 500 Internal Server Error. This is not informative and does not help the user understand what went wrong. We need to implement better error handling for this case to provide a more user-friendly error message.

Steps to Reproduce:

1. Attempt to register a new user with a username that is already taken (i.e. tristenwallace).
2. Observe that the server returns a 500 error.

Expected Behavior:

The server should return a 409 Conflict status code with a descriptive error message indicating that the username is already taken.

Actual Behavior:

The server returns a 500 Internal Server Error.

Possible Solution:

• Update the register function in the server controller to catch duplicate username errors and return a 409 Conflict status code with a descriptive error message.
• Update the frontend API error handling to display a user-friendly message when a 409 Conflict status code is received.
• Add backend route for Checking Username Availability
• Add frontend validation for Username Availability