pygrass.gis.Region.from_* should raise a ValueError if the specified map does not exist.

[pmav99]

I think it would be better if

pygrass.gis.Region.from_rast(raster_name)
pygrass.gis.Region.from_vect(vector_name)

would raise a ValueError if the specified map name cannot be found. With the current implementation, the region remains unchanged and you have no clue about that: https://grass.osgeo.org/grass77/manuals/libpython/_modules/pygrass/gis/region.html#Region.from_rast

Note, it might make sense to raise the Exception from within get_mapset_raster() but that function is used in more places and I haven't checked if this will break anything.

[neteler]

A patch would be appreciated.