so python log system can be used with sudo logsystem.
Loggers use it by default (the handler is set on the root logger).
If that is not the intent, it can be overridden explicitly.
Plugins can raise a sudo.PluginError exception to add context message
for the failure.
The callback's errstr gets filled up with the specified message.
But, as sudo expects a string constant (will not free the string),
we store it in the plugin context at least until next callback
invocation.
