Replace error with user-error where appropriate
Fix magit-annex-add for unstaged section

This commit fixes two issues with using `magit-annex-add' in the
unstaged section.

1. It was limited to adding the entire section of unstaged changes. It
   couldn't add a single file or a region selection of files.
2. If calling it on the entire seciton, all files, including untracked,
   were being added.