schema: fix typos in error messages

1. Error messages in dirreader should mention dirreader instead of
   filereader.
2. Remove \n at the ends of error messages.
3. Use errors.New for plain text error messages.
4. Give precise static-set information in one error message.

Change-Id: Iae0f46bca492e9dbbe41b4c1bebf5f5950d722b5