Suggest fabrik targets from target-not-found and wrong-kind errors

When you ask fabrik to build a target that does not exist, or to use the wrong kind for an existing target, the error now points you at the discovery command: “Run fabrik targets”. This gives you an immediate next step instead of a dead-end error when a target name was a typo or when you used the wrong verb (such as exec on a binary that is actually a library).