codegen: Free errors after returning them on the GDBusMethodInvocation
codegen: Free errors after returning them on the GDBusMethodInvocation

Those are being leaked otherwise.

Fixes https://gitlab.gnome.org/GNOME/vala/issues/657

GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala