Store a log of errors in the Client.

Store a log of errors in the Client.

Each entry is a ClientError and a datestamp. But errors generated in
the client don't _automatically_ get added to this log: they get
passed back to the caller of an individual Client method, the same as
they always have. That way the caller can decide whether the error is
one it can handle in some other way, and only if all else fails, call
client.add_to_error_log().

The next plan is for every client.add_to_error_log() to be followed by
throwing the TUI into the Error Log file.