[GRASS-git] [OSGeo/grass-addons] c9a8d6: r.vif.py: add error message

[Paulo van Breugel]

  Branch: refs/heads/ecodiv-patch-r-vif-1
  Home:   https://github.com/OSGeo/grass-addons
  Commit: c9a8d6957f5dabe0935fc9c7eef978a42fbc79c9
      https://github.com/OSGeo/grass-addons/commit/c9a8d6957f5dabe0935fc9c7eef978a42fbc79c9
  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. Print error messages in color.