Added DownloadStatus convenience

[julianrex]

PRs must be submitted under the terms of our Contributor License Agreement CLA.

Fixes: < Link to related issues that will be fixed by this pull request, if they exist >

Pull request checklist:

• Briefly describe the changes in this PR.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality. If tests were not written, please explain why.
• Add example if relevant.
• Document any changes to public APIs.
• Apply changelog label ('breaking change', 'bug 🪲', 'build', 'docs', 'feature 🍏', 'performance ⚡', 'testing 💯') or use the label 'skip changelog'
• Add an entry inside this element for inclusion in the mapbox-maps-ios changelog: <changelog></changelog>.
• Update the migration guide, API Docs, Markdown files - Readme, Developing, etc

Summary of changes

User impact (optional)

[macdrevx]

This is confusing. If this initializer is for use with download state pending, why would you ever provide an error if that's only for the failed state?

[macdrevx]

Pausing my review. I think the test improvements are the most important. Remodeling DownloadStatus as an enum can be deferred, since that seems outside of the scope here (refining an existing construct vs wrapping it in a new one).