Improve error message when icon is not found

Author: probonopd

appimagetool.c

@@ -605,7 +605,11 @@ main (int argc, char *argv[])
         } else if(g_file_test(icon_file_xpm, G_FILE_TEST_IS_REGULAR)) {
             icon_file_path = icon_file_xpm;
         } else {
-            fprintf (stderr, "%s{.png,.svg,.svgz,.xpm} not present but defined in desktop file\n", icon_name);
+            fprintf (stderr, "%s{.png,.svg,.svgz,.xpm} defined in desktop file but not found\n", icon_name);
+            fprintf (stderr, "For example, you could put a 256x256 pixel png into\n");
+            gchar *icon_name_with_png = g_strconcat(icon_name, ".png", NULL);
+            gchar *example_path = g_build_filename(source, "/usr/share/icons/hicolor/256x256/apps/", icon_name_with_png, NULL);
+            fprintf (stderr, "%s\n", example_path);
             exit(1);
         }