Aggregate all errors on config or schema refs issues

**Is your feature request related to a problem? Please describe.**
Currently when there is an issue with a `ref` then the first found is returned even if there are multiple issues of the same nature. In case there are multiple issues, many requests will be needed in order to fix all of them instead of understanding everything that's wrong with one request.

**Describe the solution you'd like**
Collect all errors of the same nature and throw them as a group.

**Describe alternatives you've considered**
This may be "solved" by the use of the `UI` which is supposed to be the main way the users will deal with `schemas` and `configurations`.

Start by locating the config and schema reference validation entry points in config-server and trace where the first error is returned. Check whether existing tests cover multiple invalid refs; done means one request reports all errors of the same nature together rather than stopping at the first.

Written by the indexing model from the issue text.