pkg_install ignores explicit version when that version does not exist

[msteijaert]

pkg_install() allows to install a specific version of a package. E.g., pak::pkg_install("data.table@1.15.2") will install data.table version 1.15.2.
However, when I specify a non-existing version such as pak::pkg_install("data.table@1.15.1"), it does not complain, but installs the latest version instead (currently 1.15.4). I think it would be useful to throw an (optional) error or warning here.