Use common error format and function for building errors

[mfelsche]

Describe the problem you are trying to solve

We currently create error events in onramp/offramps mainly for making linked transport usable and have more observability of internal errors (e.g. codec failures). But we do not necessarily follow a common format and do not use common codepaths to create errors.

Describe the solution you'd like

We should document and follow a common format and create common functions to create error events.
This will help users handle errors. Maybe a separate section about handling errors and connecting the err ports for getting good feedback is also helpful.