Incorrect error on sign-up page when user already exists

## Bug description

A clear and concise description of the bug.
Issue: In Developer Portal on complete sign up page, if a user provides an already existing user's ID, it throws the following error message: "Server error. Unable to send request. Please try again later." But we can see in the response body the appropriate error:

{"error":{"code":"ValidationError","message":"One or more fields contain incorrect values:","details":[{"code":"IdentifierAlreadyInUse","target":"email","message":"User with specified identity already exists"}]}}

The error on the page should be fixed.

## Reproduction steps

1. Go to 'Sign up'
2. Fill the inputs with existing user data
3. Click on 'Sign up'
4. See error


## Expected behavior
1. Go to 'Sign up'
2. Fill the inputs with existing user data
3. Click on 'Sign up'
4. See error
"User with specified identity already exists"

## Is self-hosted portal?

Both

## Release tag or commit SHA (if using self-hosted version)

2.32

## API Management service name

[e.g., contoso-api]

## Environment

- Operating system: [e.g., iOS]
- Browser: [e.g., Google Chrome, Safari]
- Version: [e.g., 22]

## Additional context

Message should be shown from error response from the backend
Example:
from this response error
{"error":{"code":"ValidationError","message":"One or more fields contain incorrect values:","details":[{"code":"IdentifierAlreadyInUse","target":"email","message":"User with specified identity already exists"}]}}

expected error message: "User with specified identity already exists"

Start by reproducing the Developer Portal sign-up flow with an existing user ID and inspect how the response body is handled on the complete sign-up page. Trace the handling of the IdentifierAlreadyInUse detail and verify that the page displays "User with specified identity already exists" instead of the generic server error.

Written by the indexing model from the issue text.