LDAP Login when email already exists fails silently. #2048

Describe the bug
When two users share the same email address in OpenLDAP, the first user is able to auto-generate their bookstack account. When the second user attempts to log in (with the same email address as the first user), no error message, auto-redirect to the login page.

Steps To Reproduce
Steps to reproduce the behavior:

1. Setup LDAP Auth (Using OpenLDAP)
2. Create 2 users with the same email address
3. Attempt to log into Bookstack as both users.

Expected behavior
An error message or indication of failure.

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): 0.28.3
• PHP Version: 7.3
• Hosting Method (Nginx/Apache/Docker): Apache