REFAC: add warning messages when geoid or dem cannot be opened and don't try...

REFAC: add warning messages when geoid or dem cannot be opened and don't try to use a null dataset to create the shifted dataset when the geoid cannot be opened
19 jobs for !881 with 2221_clean_of_elevation_parameters in 83 minutes and 24 seconds (queued for 3 seconds)
merge request