Azure pipeline: report failure in prepare-build step
    
    The azure pipeline defines a "prepare build" step that
    installs the FreeIPA development dependencies but the
    step does not report failures of the dnf builddep command.
    
    As a consequence, subsequent steps may fail (for instance
    because of components not installed such as tox) but are
    hard to diagnose.
    
    The fix reports the command failure.
    
    Fixes: https://pagure.io/freeipa/issue/8022
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>