58fe997 Raise a ValueError with details if module not found in PDC.

Authored and Committed by ralph 6 years ago
    Raise a ValueError with details if module not found in PDC.
    
    Currently, this will fail two lines later when we try to access
    `pdc_module['modulemd']` with an unhelpful `TypeError` since
    `pdc_module` is `None`.
    
    Signed-off-by: Ralph Bean <rbean@redhat.com>