Update error messages

[daniellemaxwell]

Scope of changes

Display error message when a user attempts to add a VASP with an existing domain. The change also fixes an error that didn't display message to unverified users on the /member page.

Type of change

• [x] bug fix
• [ ] new feature
• [ ] documentation
• [ ] other (describe)

Acceptance criteria

https://www.awesomescreenshot.com/video/24253864?key=40235ff596a1051de518574bd1995444

Author checklist

• [x] I have manually tested the change and/or added automation in the form of unit tests or integration tests
• [ ] I have updated the dependencies list
• [ ] I have recompiled and included new protocol buffers to reflect changes I made
• [ ] I have added new test fixtures as needed to support added tests
• [x] Check this box if a reviewer can merge this pull request after approval (leave it unchecked if you want to do it yourself)
• [x] I have moved the associated Shortcut story to "Ready for Review"

Reviewer(s) checklist

• [ ] Toasts display error messages
• [ ] Any new user-facing content that has been added for this PR has been QA'ed to ensure correct grammar, spelling, and understandability.