Allow users to skip permission errors

Created by: malomarrec

Current behavior:

If the spec includes a repository I don't have read access to, the entire preview execution fails. To reproduce:

• Create a spec with a named list of repositories using on.repository.
• Include a repository you don't have permission to read.
• Run src batch preview with the --skip-errors flag

Expected behavior:

On src-cli*: If you use --skip-errors, repositories to which you don't have read access are skipped, with a warning about the reason. That does not fail the entire preview, just skips the repository, with a warning. On SSBC: same behaviour as above by default.