Add more error prefixing when parsing commit objects

Add more error prefixing when parsing commit objects

I've got more debug information in the error case that motivated
https://github.com/ostreedev/ostree/pull/2884/commits/bae4347abeaa2a66d213758f790058f42cb71fd1
"pull: Add error prefixing for corrupt checksums"
where the sole error is

`error: Invalid checksum of length 0 expected 32`

This must be coming from the pull code in the case where we've
already fetched the commit object.

- Add some error prefixing here in the core commit validation code
- Ensure that we do the validation immediately after loading, including
  of the parent commit reference where I think this error must be coming
  from
- Then the pull code can just safely call `ostree_commit_get_parent`
  which already does the hex conversion etc.