refactor: use multierror in config validation

What does this MR do?

Example:

FATA[0000] invalid config settings                       error="2 errors occurred:\n\t* no listener defined, please specify at least one --listen-* flag\n\t* auth-redirect-uri must be defined if authentication is supported\n\n"

Closes #665 (closed)

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines