model: align error with ECS

[axw]

The ECS Error definition only defines a minimal set of values. There is no concept for chained exceptions. There is no description how to relate error logs.

APM uses an extensive fieldset in the error namespace, with the main sub-namespaces being error.exception, error.log and error.grouping_key. We should propose changes (or adapt our model) to ECS.

We should take into account #3877 while implementing this.