restore find-jar error detection.

lost with 98882a40062a2a7c07f3cf5b43c5a41c9a93c25ec
on deadbranch-1.124.2 (DEVEL for 6.0 merged in 1d6ec01d1c4bc3d763d7255518f833cbcf29ecc8)
originally added in 3bd96a5ee3dac23618cef1cee09bfa001b06ef0b
find-jar at build time, not at package prep
cleanup unused params