get_all_versions doesn't fail gracefully when it hits a private object

[amoeba]

> library(arcticdatautils)
> env <- env_load('production')
> get_all_versions(env$mn, 'doi:10.18739/A2T67B')
Your login certificate is missing or unreadable. You must login again via CILogon using 'dataone::downloadCert(CertificateManager())'
Attempting to call as public user without being authenticated.
Your login certificate is missing or unreadable. You must login again via CILogon using 'dataone::downloadCert(CertificateManager())'
Attempting to call as public user without being authenticated.
Your login certificate is missing or unreadable. You must login again via CILogon using 'dataone::downloadCert(CertificateManager())'
Attempting to call as public user without being authenticated.
Error in get_chain_neighbors(node, pid) : 
  trying to get slot "obsoletes" from an object of a basic class ("NULL") with no slots
In addition: Warning message:
In .local(x, ...) :
  Error getting SystemMetadata: READ not allowed on arctic-data.9827.4