[MINOR] adjust error messages about conflicting proxies

[MINOR] adjust error messages about conflicting proxies

It's not easy to report useful information to help the user quickly
fix a configuration. This patch :
  - removes the word "listener" in favor of "proxy" as it has been
    used since the beginning ;

  - ensures that the same function (hence the same words) will be
    used to report capabilities of a proxy being declared and an
    existing proxy ;

  - avoid the term "conflicting capabilities" in favor of "overlapping
    capabilities" which is more exact.

  - just report that the same name is reused in case of warnings