Add a new error code to allow ra_svn to signal whether tunnel creation failed.

Use this error code in the 'svn' client to remind users of the fact that
the --username and --password options do not work as they might expect
when using svn+ssh:// URLs.

This is a common question in the #svn IRC channel.

* subversion/include/svn_error_codes.h
  (SVN_ERR_RA_CANNOT_CREATE_TUNNEL): New.

* subversion/libsvn_ra_svn/client.c
  (make_tunnel): If the tunnel cannot be created, raise the new error code.

* subversion/svn/svn.c
  (sub_main): If tunnel creation failed and the user passed --username or
   --password, assume the user is trying to use svn+ssh:// improperly and
   issue a warning.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464407 13f79535-47bb-0310-9956-ffa450edef68