List Editor: Improve validation error handling

[dwaring87]

Description

This adds small improvements to the error handling for list validation in the list editor:

• Show an error message before validation if the list type has not be selected first
• Show an error message if there is a server-side error while validating the list

Checklist

• [ ] Refactoring only
• [ ] Documentation only
• [ ] Fixture update only
• [ ] Bug fix
  • [ ] The relevant issue has been closed.
  • [ ] Further work is required.
• [ ] New feature
  • [ ] Relevant tests have been created and run.
  • [ ] Data was added to the fixture
  • [ ] Data was added via a patch in /t/data/fixture/patches/.
  • [ ] User-Facing Change
  • [ ] The user manual in /docs has been updated.
  • [ ] Any new Perl has been documented using perldoc.
  • [ ] Any new JavaScript has been documented using JSDoc.
  • [ ] Any new legacy JavaScript has been moved from /js to /js/source/legacy.