[GRASS-git] [OSGeo/grass-addons] 671b83: r.vif.py: add error message

[Paulo van Breugel]

  Branch: refs/heads/ecodiv-patch-r-vif
  Home:   https://github.com/OSGeo/grass-addons
  Commit: 671b83deff5f80b73e3781fa4b512fc1bf1d0e9c
      https://github.com/OSGeo/grass-addons/commit/671b83deff5f80b73e3781fa4b512fc1bf1d0e9c
  Author: Paulo van Breugel <paulo at ecodiv.earth>
  Date:   2023-09-16 (Sat, 16 Sep 2023)

  Changed paths:
    M src/raster/r.vif/r.vif.py

  Log Message:
  -----------
  r.vif.py: add error message

The error message given when one of the input layers has no values (within the computational region) does not help to understand the problem. Added an error message to suggest the possible cause of the error.