wscript: remove usage of ctx.options.with_tester

This option no longer exists.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Simplified signature of MessageHandler delegate

This delegate has target implicitly. Also, adaptor simplified to a
struct.

Examples updated.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Throw error in MessageHandler

 - Used to return boolean value.
 - Also remove create_error() helper.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add API to serialize GLib.Error

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Serialize and propagate MessageErrors

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>