Improve message of check 0797

[leendertvanwolfswinkel]

Current (schema 219):

0797 | Warning | The file in v2_global_settings.dem_file has no EPSG code or the EPSG code does not match does not match v2_global_settings.epsg_code

New text (text should also be updated to schema 300):

The file in v2_global_settings.dem_file has no EPSG code or the EPSG code does not match does not match v2_global_settings.epsg_code. Reproject the DEM, or reassign the projection to the DEM, or fill in the correct EPSG code in v2_global_settings.epsg_code.

[margrietpalm]

@leendertvanwolfswinkel part of this ticket cannot be implemented because model_settings.epsg_code will be removed. There should still be a check to ensure that the dem file has an epsg code (and what to do if this is the case).

Mismatches will be handled by https://app.zenhub.com/workspaces/team-3di-5ef60eff1973dd0024268b90/issues/gh/nens/threedi-modelchecker/415