[Front]Check that the email does not already exist when creating a user (on save)

[brauliodiez]

Step 1: Implement in the mock server on the backend.

Step 2: When the error comes from the server (409 on error) upon clicking save, display the error message.

export const enum CustomInternalCodes {
    emailAlreadyRegistered: 'user-001',
}

export type Severity = 'error' | 'critical';

{
 error: CustomInteralCodes,
 severity: Severity;
 message? string;
}

Check with @manudous which toast notification to use.