Incorporate improved error message from PR for osdf-client

[jhiemstrawisc]

A pull request from the osdf-client failed to make it into both the old osdf client and the new pelican. It can be found at htcondor/osdf-client#78, and it deals with improving error messages w.r.t. upload failures.

It doesn't look like the structure of the various files touched in the PR has changed too much between osdf-client and pelican, aside from a few sections of code moving to new files, so this should be an easy one to slip in.