pmb.helpers.ui: Add must_exist argument to check_option() + pmb.config.other: Don't require selected UI package to exist

Without this, $ pmbootstrap init crashes if a nonexistent UI package is selected. This can happen if one previously selected a UI package in a former checkout of pmaports, but then switched away to a different one that doesn't have the UI package. For example, if someone is working on a new UI in a feature branch and then switches away from it, or if a UI is removed from the canonical pmaports repository altogether.