Interpolation errors are not logged by benthos when it fails

[mannharleen]

consider the below config:

  processors:
    - log:
        level: DEBUG
        message: '${! content() + meta() }'

The above will fail; meta().string() should be used instead.

Expected: benthos should log an error with the appropriate message Actual: Instead of logging an error, the log processor actually logs an empty msg, like below:

DEBU                                               @service=benthos label="" path=root.pipeline.processors.0