Xgit.Tag.from_object/1 should return error if object type is not recognized.

[scouten]

Changes in This Pull Request

Bug fix: Xgit.Tag.from_object/1 would return a Tag structure with type: :error if the type field was present, but invalid, in the tag object.

Updated to return {:error, :invalid_tag} instead.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented. n/a
• There is test coverage for all changes.
• All cases where a literal value is returned use the cover macro to force code coverage. n/a
• Any code ported from jgit maintains all existing copyright and license notices. n/a
• If new files are ported from jgit, the path to the corresponding file(s) is included in the header comment. n/a
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment. n/a