ref-filter: modify error messages in `grab_objectname()`
As we plan to use `grab_objectname()` for `tree` and `parent` atom,
it's better to parameterize the error messages in the function
`grab_objectname()` where "objectname" is hard coded.
Mentored-by: Christian Couder <chriscool@tuxfamily.org> Mentored-by: Heba Waly <heba.waly@gmail.com> Signed-off-by: Hariom Verma <hariom18599@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>