Add the concept of warning/error codes

[nfrechette]

ACL has many warnings and errors added to the output log. To help searching for what they mean, a consistent naming scheme should be introduced.

E.g: [ACL_ERROR_02] - Track index %u is queried explicitly at runtime but can been stripped if equal to the bind pose. Make sure this bone is excluded or results will be incorrect!

This would allow someone to quickly google for the error code or to search the documentation for it. It would also allow us to document all error codes explicitly.