Not useful anymore (guess they were when those layers were created).
Much shorter stream of messages on module init now.

    ag -l Copyright | xargs sed -i \
        "s/\(.*copyright (C) [0-9]\+\)\(-[0-9]\+\)\?\(.*\)/\1-$(date +%Y)\3/I"

Pass around the module instead of its dict (getting the latter is fast
if needed), mark function raising with negative results, check all errors,
consistent names...

Close #456

Store the encode/decode functions for the right codec in the connection.
The Python encoding name has been dropped of the connection to avoid the
temptation to use it...

This makes the Diagnostics independent from further operations on the cursor
and allows using it with exceptions not generated by a cursor.