(MODULES-8171) Run fails if paths in $env:lib don't exist
(MODULES-8171) Run fails if paths in $env:lib don't exist

  Prior to this commit, Puppet fails while compiling C# code within init_ps.ps1 with a cryptic no implicit conversion of nil into String message.

  This commit checks that the LIB environment variable contains invalid paths and fails with a useful error message.

puppetlabs/puppetlabs-dsc,puppetlabs/puppetlabs-dsc,puppetlabs/puppetlabs-dsc