Error when redirect from/to collisions exist at build time (again)

[Krinkle]

This follows-up issue https://github.com/jekyll/jekyll-redirect-from/issues/164, which was closed in favour of https://github.com/jekyll/jekyll/pull/8342, which was fixed and released in Jekyll 4.2.0.

Unfortunately, it seems the chosen solution in Jekyll core did not address #164 in the jekyll-redirect-from. As of jekyll 4.3.3 and jekyll-redirect-from 0.16.0, conflicting redirects are still silently overwriting each other at build time.

\cc @pathawks @SeekingMeaning @ashmaroli @DirtyF