Warn when logging buffer is full

[stephane-caron]

As discussed with @ubgk today, currently we don't check that logging succeeds at every spine cycle:

vulp/vulp/spine/Spine.cpp

Line 72 in af23b12

logger_.put(working_dict_);

The output from mpacklog::Logger::put is a success boolean. When it fails we should warn users that some log entries are being lost. (Perhaps suggest follow-up actions such as (1) increasing the size of the circular buffer or (2) reducing the size of logged dictionaries.)