Reading a token from the keyring can sometimes fail for reasons
other than the token not being present in the keyring. This PR
introduces a new error used to indicate that the token requested
could not be found in the keyring and propagates errors from the
keyring up to the caller, so that the caller can act on them.
Additionally, this fixes a bug in the client cache where it would
incorrectly assume that a token had been removed from the
keyring, when in fact there was an error reading from the keyring.
Johan Brandhorst-Satzkorn
1a6320794e