Unknown Client error on call to issuer

[jhiemstrawisc]

Some of the recent churn with the SciTokens issuer saw the deletion of some client configs at the CHTC issuer. When my local client attempted to get a token from it, the issuer responded with a 400 Bad Request, pointing to an unknown client, with no additional information about how to troubleshoot.

The solution was to delete the file ~/.config/osdf-client/oauth2-client.pem, but I don't know that I would have figured that out in a reasonable amount of time had @bbockelm not pointed me in that direction. We should look at handling this error more explicitly so that users can figure out workarounds on their own.