Set an error message when keyring get_princ fails

Set an error message when keyring get_princ fails

When attempting to use a keyring cache that doesn't exist, set an error
message when we fail to read a principal name, as we do when we return
the same error code when using a file ccache.

[ghudson: removed unnecessary check for d->name nullity.]

(cherry picked from commit c25fc42e8eac7350209df61e4a7b9960d17755ca)

ticket: 7809