Better message when `DOWNLOAD_TARGET = NOP`

See https://github.com/SynoCommunity/spksrc/commit/89b235855839174913256a0d0fb5c459b625dd0a#diff-de37b3c0178feefc04a02efe9c69d2deL5.
Merge pull request #792 from Dr-Bean/patch-1

Better message when `DOWNLOAD_TARGET = NOP`