- Map errors in `fetch_html` to include the source url
- Change `article_link` to `article_source`
- Add `Into` conversion for `UTF8Error`
- Collect errors in `generate_epubs` for displaying in a table
Using this custom error type, many instances of unwrap are replaced
with mapping to errors that are then logged in main.rs. This allows
paperoni to stop crashing when downloading articles when the errors
are possibly recoverable or should not affect other downloads.
This subsequently introduces ignoring the failed image downloads
and instead leaving the original URLs intact.
