Low: xml.c: better info in cli_config_update on no validation success

This commit turns [error message] from testcase at 35fadfd commit's
message from

Your current configuration could only be upgraded to pacemaker-1.0...
the minimum requirement is pacemaker-2.0.

into

Your current configuration could not validate with any schema in
range [pacemaker-1.0, pacemaker-2.6]