Bug 29857: (QA follow-up) Fix unit test Object.t

Bug 29857: (QA follow-up) Fix unit test Object.t

Unit test failling with :
    #   Failed test 'Exception message correct'
    #   at /kohadevbox/koha/t/db_dependent/Koha/Object.t line 335.
     #          got: 'Exception 'Koha::Exception' thrown 'Asked to embed items but its return value doesn't implement to_api'
    # '
    #     expected: 'Asked to embed items but its return value doesn't implement to_api'
     # Looks like you failed 1 test of 29.

Fixed by using $@->message like in other places.

prove t/db_dependent/Koha/Object.t

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>