Fix error handling in payload copying

Fix error handling in payload copying
- Lift the payload copy into a separate helper function to clean
  things up a bit
- The code to handle read errors was unreachable before this,
  causing any read error (rare as they might be) to be silently
  ignored and package generation considered successful.
- This could/should perhaps use ufdCopy() instead but that ha
  its own set of problems (not clean wrt large files, error reporting...)