Fix upgrading the errordocs directory in rb-site.
Review Request #8925 — Created May 8, 2017 and submitted
rb-site was attempting to create a new errordocs directory for the new 500.html page. However, this actually failed when upgrading, since there was already an errordocs symlink which then got removed after attempting to create the directory. This meant that when we tried to write to the new directory, it failed. A second upgrade attempt would work, since the symlink was gone and the directory creation worked. We now attempt the directory creation after removing the legacy symlink, ensuring that upgrades work right the first time.
Upgraded an existing site with the symlink. The upgrade worked.