enhance perfcheck failing message

add the expected number

Change-Id: Ib068288f7efbfebc2705c6084dc34a651def1986
Reviewed-on: https://gerrit.libreoffice.org/11948
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>