Pass along bloblang/yaml error context

[abergmeier]

We are trying to report Benthos errors to Google Cloud Error Reporting. There they want to have a https://cloud.google.com/error-reporting/reference/rest/v1beta1/ErrorContext#SourceLocation.

For that it would be great if, once an error occurs meta is extended by at least:

1. yaml name
2. line number

For the functionName it might make sense to use processor label.