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.
This change allows for parallel downloads of HTML pages upto a maximum
number of concurrent HTTP requests which is more efficient than
before where all HTTP requests are likely to begin at the same time.
