Devfile Gem: Handle devfile gem validation errors as UI errors instead of 500 exceptions

MR: https://gitlab.com/gitlab-org/remote-development/devfile-gem/-/merge_requests/42+

This is the issue representing the devfile gem work for Rails: Handle devfile gem validation errors as ... (#419476 - closed)

See Rails: Handle devfile gem validation errors as ... (#419476 - closed) for more details.

Availability and Testing

Ensure updated test coverage in unit/integration/feature tests.