Propagate ValidationObserver's errors added via setErrors

[Borzik]

This PR enables error propagation, so calling setErrors on parent (form-level) observer will be enough and errors will be rendered even on providers which are children of nested observers.

Example of issue here: https://codesandbox.io/s/veevalidate-30-manually-setting-errors-iknf0

This will be quite useful for those who have such structure:

ValidationObserver(ref="form" tag="form")
  Tab
    ValidationObserver
      ValidationProvider
        input
  Tab
    ValidationObserver
      ValidationProvider
        input

Current implementation requires you not only to call setErrors on parent observer, but also on each nested ValidationObserver, so snippet like this has to be added every time:

this.$refs.form.observers.forEach((observer) => {
  observer.setErrors(response.data);
});

Note that this snippet is for one-level nesting. I'm not sure if anyone would need to have ValidationObserver > ValidationObserver > ValidationObserver, but then the code will be even more complicated.

If you think that it's better to keep setErrors simple, maybe another method should be added for this feature, I can make another PR.

[logaretm]

Looks good to me, thanks!