Fix error handling in user magament in the Console

[ryaplots]

Summary

This PR fixes an issue spotted by cybereye here: https://github.com/TheThingsIndustries/lorawan-stack-support/issues/1192#issuecomment-2538587453

Changes

• Add feature check to add button
• Fix form not handling errors

Testing

Steps

1. Log in with a user that does not have user create right and go to the user management page - there should be no add user button.
2. Log in with a user that has the right to create users and try to add a user with an existent id - there should be an error notification in the form.

Results

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.