Provide more better logging for keyring expiration exception

[Barry Warsaw]

Keyring .xz files have a .json file containing some information, including an optional `expiry` key which describes the datetime (in UTC) at which the key expires. If this expiration occurs, you get an exception in the log file like so:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 129, in __next__
    step()
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 275, in _get_channel
    self.blacklist)
  File "/usr/lib/python3/dist-packages/systemimage/keyring.py", line 142, in get_keyring
    raise KeyringError('expired keyring timestamp')
systemimage.keyring.KeyringError: expired keyring timestamp

IWBNI this contained some additional information such as:

* Which keyring is expired (but look higher up in the log file for that information)
* The value of the expiry key
* The current timestamp (probably available in the log file timestamp)