Sometimes when value can't be casted to expected type we need to deep dive into the problem and now there is no good way to know even which column caused the exception.
All we get is this:

In CastingContext.php line 39:
                                         
  [Flow\ETL\Exception\CastingException]  
  Can't cast "NULL" into "string" type   
                                         

We should improve the readability of this exception point out not only what can't be casted but ideally also which column caused it. Would be nice to also attach raw serialized row but it might not be possible without bigger refactoring.