wp theme mod set prints its failure message as Success and exits 0 #570

When wp theme mod set cannot save the value, it prints the failure through WP_CLI::success():

$ wp theme mod set background_color 123456
Success: Could not update theme mod background_color.
$ echo $?
0

To reproduce, add a pre_set_theme_mod_background_color filter that returns a different value (for example from a mu-plugin). Theme_Mod_Command::set() reads the mod back, sees it does not match, and calls WP_CLI::success( "Could not update theme mod {$mod}." ). Scripts that check the exit code treat the failure as a success.

Expected: Error: Could not update theme mod background_color. with exit code 1.