[publication] No error message for duplicate investigator email

[CamilleBeau]

Describe the bug
If you are proposing a project with the same lead investigator email as another project, but different name, it fails without any error message. Nothing happens when you click submit, but the project is not proposed and there is a 500 error.

To Reproduce
Steps to reproduce the behavior (attach screenshots if applicable):

1. Go to publication module with the "Propose a project" permission
2. Click on propose a project
3. Fill out the form properly, but put an existing investigator email with a different name
4. Click submit
5. See error

What did you expect to happen?
An error message should show with a description of what went wrong

Browser Environment (please complete the following information):

• OS: [e.g. Plan9, HURD, Oberon, etc]
• Browser [e.g. chrome, firefox] (note that only these two browsers are supported)
• Version [e.g. 22]

Server Environment (if known):
Note: We only support the most recent release of LORIS.

• LORIS Version: [e.g. 22.0.0]
• Linux distribution and Version: [e.g. Ubuntu 16.04, CentOS 7]
• MySQL/MariaDB Version: [e.g. MySQL 5.7, MariaDB 10.3]

Additional context
Add any other context about the problem here.