create_image()

When convert failed, the error messsage would look like:

  create btrfs filesystem:
      blocksize: 4096
      nodesize:  16384
      features:  extref, skinny-metadata (default)
  creating ext2 image file
  ERROR: failed to create ext2_saved/image: -1
  WARNING: an error occurred during conversion, filesystem is partially
  created but not finalized and not mountable

We can only know something wrong happened during "ext2_saved/image" file
creation, but unable to know what exactly went wrong.

This patch will add the following error messages for create_image() and
its callee:

1) Sanity test error
2) Csum calculation error
3) Free ino number allocation error
4) Inode creation error
5) Inode mode change error
6) Inode link error

With all these error messages, we should be pretty easy to locate the
error without extra debugging.

Reported-by: Serhat Sevki Dincer <jfcgauss@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>