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.

src/raster/r.vif/r.vif.py

@@ -123,6 +123,9 @@
 
 
 # Functions
+def prRed(skk): print("\033[91m {}\033[00m" .format(skk))
+
+
 CLEAN_RAST = []
 
 
@@ -348,7 +351,12 @@ def main(options, flags):
                     vifstat = compute_vif2(x, y)
                 else:
                     # Compute vif using sample
-                    y = p[:, k]
+                    try:
+                        y = p[:, k]
+                    except Exception as e:
+                        prRed(f"An exception occurred: {str(e)}")
+                        prRed("Tip: check if all input rasters have values"
+                              " within the computation region.")
                     x = np.delete(p, k, axis=1)
                     vifstat = compute_vif(x, y)