This issue has been addressed too early in the process lifecycle. The line number should be incremented later, say in the view layer when the error is displayed. A simple helper to determine what the index increment should do well.

$row + $indexIncrement

This is so we don't have to rebuild the array, which is computationally expensive for large datasets and could be frustrating for a developer expecting untouched index values, or likewise needing to prepare our special format.