Resolve incorrect exit code from svnauthz when testing a non-existant url.

* subversion/include/svn_repos.h
  (svn_repos_authz_read2): Improve the documentation to be clear that
    SVN_ERR_AUTHZ_INVALID_CONFIG is only returned for validation failures
    and not for missing files.

* subversion/libsvn_repos/authz.c
  (authz_retrieve_config_repo): Never return SVN_AUTHZ_INVALID_CONFIG since
    all of these error paths are not validation errors but some sort of  
    operational error like invalid paths.  Documentation adjusted to match.
  (authz_retrieve_config): This never returns SVN_AUTHZ_INVALID_CONFIG,
    documentation adjusted to reflect this.

* subversion/tests/cmdline/svnauthz_tests.py
  (svnauthz_validate_repo_test, svnauthz_compat_mode_repo_test): Adjust
    the expected return code for the two tests that test non-existant urls.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1440023 13f79535-47bb-0310-9956-ffa450edef68