[web server] #1642893 issue a 403 instead of a 500 on Unauthorized exception
To do so, we allow an arbitrary code to be passed to ``error_handler``. Default
error code is still 500.
ifnotrql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',ask_confirm=False):rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',ask_confirm=False)