Don't show login errors when logged in successfully

[dparker1005]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Makes sure that "log in errors" aren't shown when a user is successfully logged in.

Resolves XXX.

How to test the changes in this Pull Request:

Couldn't replicate, but here were the reported steps.

1. Go to your frontend login page
2. Enter a username but no pw, submit form
3. Shows error message Error: The password field is empty.
4. add your PW, submit form.
5. you get logged in, but error message still showing.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.