Improve Error Messaging for Login/Register Process

[runleveldev]

• When a user registers, duplicate key errors on username/email should flash an error that user is already registered.
• On login, a temporary banner should be displayed while the backend waits for the 2FA push notification response.
• Instead of the generic "Authentication request was denied error" from a failed 2FA push notification, the error message flashed to the user should specify notification denied or notification timed out.