Different status messages are shown if email address already existed

Problem/Motivation

If I attempt to create an account for a new user that doesn't exist, after registration, I'm shown a message on the log in screen indicating that a welcome message was sent to my address. But if I attempt to create an account for a user that already exists, then no status message is shown on the log in screen.

Steps to reproduce

1. Go to /user/register and register a new user account
2. Observe the status message shown after submitting the form
3. Fill out the user registration form again using the same username or email address

Proposed resolution

Copy the messages from core to conceal what actually happened on form submission.

Limitation:
If the Drupal site is not set up to require email address verification, users will still get a different result. But those sites are probably not using this module.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A