Creating a tag with spaces returns a vague error

Summary

Tag names don't accept spaces, we should either:

• Make this clear to users in the error message
• Auto snakeize the tag name like we do with project names (ie "a test tag" -> "a-test-tag")

Steps to reproduce

1. Navigate to the new tag form: https://staging.gitlab.com/gitlab-org/gitlab/-/tags/new
2. Specify a tag name with spaces
3. Submit

Relevant logs and/or screenshots