Bad validation message/flow for new milestone form

Summary

Creating a new milestone has a bad validation message or bad validation flow.

Steps to reproduce

On GitLab.com

1. Click to go to the new milestone form page.
2. Leave the Projects field empty.
3. Click "Create Milestone"

Expected behavior

1. A validation message that makes more sense, e.g. Please select at least one project.

OR

2. A better alternative is to leave the Create Milestone button disabled until the required fields are all entered. In this case, the Projects field is a required field.

Actual behavior

See screenshot.

Relevant logs and/or screenshots