Sinks: Insert sink validation and error cause

[manrodrigues]

When a sink turns into an error it is impossible to the user to know why it is happening.

We need to insert a help message that explain to the users what cause the failure (as we do to the policies that fails to apply).

It is fundamental for orb users since once that sinks fails, no data is published on grafana/prometheus

[manrodrigues]

We have the error returned on sinks response now