[GRASS-git] [OSGeo/grass-addons] 176bf2: r.vif.py: add error message (#942)

[Paulo van Breugel]

  Branch: refs/heads/grass8
  Home:   https://github.com/OSGeo/grass-addons
  Commit: 176bf29887907dbd9b7e54de4a14f4189c243d90
      https://github.com/OSGeo/grass-addons/commit/176bf29887907dbd9b7e54de4a14f4189c243d90
  Author: Paulo van Breugel <paulo at ecodiv.earth>
  Date:   2023-09-18 (Mon, 18 Sep 2023)

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

  Log Message:
  -----------
  r.vif.py: add error message (#942)

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. Print error messages in color.