tach show should warn if there are no declared dependencies in the tach.toml

[emdoyle]

Often, a new user will not realize that they need to run tach sync to auto-discover their pre-existing dependencies before running tach show --web. This results in a confusing experience looking at a bunch of disconnected boxes.

Similar to an existing check in tach sync, we should show a warning and suggestion when we detect that no dependencies have been declared. In fact, it's probably best to fail in this case by default, and only allow bypassing it with something like --force (although even this may not be necessary).