libgis: do_spawn() notify which command failed and why

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@74061 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/gis/spawn.c

@@ -686,7 +686,7 @@ static int do_spawn(struct spawn *sp, const char *command)
 	do_bindings(sp->bindings, sp->num_bindings);
 
 	execvp(command, (char **)sp->args);
-	G_warning(_("Unable to execute command"));
+	G_warning(_("Unable to execute command '%s': %s"), command, strerror(errno));
 	_exit(127);
     }