I think it would be very helpful to create an error report after a completed nest run. Primarily, I'm concerned with parts in CSVs that cannot be found/opened and parts that are too large to fit inside the required sheet (unplaced parts).
Unplaced parts in particular can be easy to miss even when manually nesting.
I agree that the error reporting on command line mode would be useful. I will make a note of it for the next major update to NestFab.
I think we do warn quite well in the nesting interface and again on export if parts have not been nested in NestFab's interactive mode but I will review this again to see if there is anything else we can do.
Depending upon how you are using NestFab overly persistent warnings about un-nested parts might be intrusive. I am thinking particularly for people experimenting and using it to generate information for quoting purposes.