Add id attribute to ValidationErrors

[mpsonntag]

The ValidationError class provides the attributes obj (odml object raising the issue), msg and rank (warning or error).

Adding an id attribute and adding ids to the available validation handlers would enable to filter the set of ValidationErrors in a validation result to specific warnings or errors.
This would help displaying only specific warning messages and test validation implementations without having to rely on the message itself.

[mpsonntag]

Closed via PR #389