Fatal errors followup

[lovromazgon]

Description

Followup to #1811 that changes a few things:

• cerrors.FatalError only wraps the error into a fatal error if the supplied error is not nil and if it's not already a fatal error (this prevents multiple fatal error: fatal error: ... messages)
  • added tests for both cases
• removed unnecessary check for in fatalError.Error, since now the Err field in fatalError is guaranteed to be populated
• removed duplicated test
• changed the DLQ node to only return a fatal error if the DLQ is actually enabled (i.e. if window nack threshold is greater than 0)
  • also removed the wrapped error message if the DLQ is not enabled
• made sure the processor node also returns a fatal error if the processor doesn't return an expected type (e.g. if it returns a nil record)

Quick checks

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.