Show the path checked on MissingComponentError

I wanted it when things went wrong, thought others might also.

cc @opscode/client-engineers 
We shouldn't need this fallback code, I only see one place where we use this exception:

```
git grep -n MissingComponentError
lib/chef-dk/component_test.rb:106:        raise MissingComponentError.new(name)
lib/chef-dk/exceptions.rb:41:  class MissingComponentError < RuntimeError
spec/unit/command/verify_spec.rb:66:    it "raises MissingComponentError when a component doesn't exist" do
spec/unit/command/verify_spec.rb:68:      expect{command_instance.validate_components!}.to raise_error(ChefDK
```

Since we're on 0.x we don't have to worry about it, but I think it's fair to treat our exceptions as internal APIs for raising.