Blender Git Loki

Alembic: addition of a simple logging class.

The idea is to have a system where we properly log error messages and
let the users know that errors occured redirecting them to the console
for explanations. This is only implemented for the exporter since the
importer already has similar functionalities; however they shall
ultimately be unified in some way.

Reviewers: sybren, dfelinto

Differential Revision: https://developer.blender.org/D2541