Enhanced error messages for btrfs-convert

This patchset can be fetched from github:
https://github.com/adam900710/btrfs-progs/tree/convert_error_messages
As usual, it's based on latest stable tag (v4.17.1).

There is one error report of btrfs-convert, the error message looks pretty meaningless:

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

After some investigation, the problem turns out to be read failure. But the error number is intermediate number (-1) returned from read_disk_extent().

This patchset will first fix the intermediate return number of read_disk_extent(), then add more error messages for btrfs-convert (at least convert part) to makes it easier to identify the problem.

In this particular case, it should output things like:

create btrfs filesystem:

      blocksize: 4096
      nodesize:  16384
      features:  extref, skinny-metadata (default)

creating ext2 image file
ERROR: failed to calculate csum for bytenr 2732765184 len 4096, Input/output error
ERROR: failed to create ext2_saved/image: -5
WARNING: an error occurred during conversion, filesystem is partially created but not finalized and not mountable

Qu Wenruo (2):
btrfs: convert: Make read_disk_extent() return more meaningful -EIO
other -1
btrfs-progs: convert: Output meaningful error messages for
create_image()

 convert/main.c      | 37 ++++++++++++++++++++++++++++++-------
 convert/source-fs.c |  2 +-
 2 files changed, 31 insertions(+), 8 deletions(-)

2.19.0