Tweak ra session open failure error generation to produce a localizable message
and use an easy to check code.

* subversion/include/svn_error_codes.h
  (SVN_ERR_RA_CANNOT_CREATE_SESSION): New error code.

* subversion/libsvn_ra/ra_loader.c
  (svn_ra_open4): Wrap provider specific error codes with generic error.
    Make message localizable.

* subversion/libsvn_ra_local/ra_plugin.c
  (svn_ra_local__open): Remove wrapping that has been in ra_loader.c for some
    versions to avoid duplicated error messages.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554027 13f79535-47bb-0310-9956-ffa450edef68