Spack packages may not have a public download option, and can implement
`download_instr` to inform users how to obtain the artifacts needed to
build. `spack checksum` however did not account for this and would print
out a confusing error message when invoked on such packages ("Could not
find any remote versions").
This PR updates the error message to output the manual download instructions
if `spack checksum` is invoked on a package with `manual_download = True`.
68e00e7073