`snap.auxdata.dem_create` Check if files listed in VRT exist

[maawoo]

Passing a VRT to dem_create that contains non-existing source files does not raise an error or at least warn the user of missing files. The resulting DEM is either just partly created or filled with nodata values if none of the source files are available. This could easily be overlooked by users and could result in incorrect results down the line if the output file is passed on to other software automatically.