Feature Request: Flagging - not uniquely geocoded

[KLBoles]

As I'm working with large sets of spatial data, I find that several of my locations are returning as not uniquely geocoded. For cleaning purposes, it would be nice for those that return as such to be flagged and then additional possible addresses to also be returned. See stackoverflow for current manual solution.

[mrhumanzee]

On a related note, it would nice if the warning message returned by geocode() indicated the index of the observation(s) that cannot be assigned a unique address. This would make flagging those observations much easier.