Some people had issues with the file that were resolved by deleting the file and allowing the downloader to re-download it. However, that didn't solve the issue for me.
After trying several times, I resolved myself to redownloading the installer, but before I did, I moved the folder, to ensure that there were no overwriting issues when downloading the fresh one. On a whim, I retried one last time, and was greeted with success; I don't know if moving the folder caused it to re-check the files, or if the fact that the installer was within the Diablo III directory (Diablo III/Diablo-III-8370-enGB-Installer, originally) caused some sort of clash, but I am now ready to charge forth to New Tristram!