Warning for command matching no packages?

I think it might be useful to issue a warning or fail if a command specifies a package spec that matches no packages in the workspace. In my case, the directory of the package didn't match the `name` field in the `package.json`

Steps to repro:
- attempt to run a command specifying a package that doesn't exist in the workspace e.g. `wsrun -p nonexistent -c build`

Expected Behavior:
- this command doesn't map to any packages in the workspace so should error (potentially in a supressable way `--allow-empty-package-list`?, names are hard 🙃 )

Actual Behavior:
- the command immediately completes, reporting success