Improve warnings when resampling or cube slices leads to (near) empty surfaces

[jcrivenaes]

While working with #1156, I see that situations where map nodes get very few are not handled with sufficient warnings.

E.g. resampling which would result in an empty map will not produce any error messages or warnings.

xtgeo/src/xtgeo/surface/_regsurf_oper.py

Line 117 in 09dadec

def resample(self, other, mask=True, sampling="bilinear"):

In addition, the routines that slices a cube with a surface returns an istat/ier, but it appears that this is not robust for unknown reasons

xtgeo/src/xtgeo/surface/_regsurf_cube.py

Line 126 in 09dadec

def _slice_cube_v2(