Make the bulk edit error messages specific to the type of error

- Before, everything would just say "Enter a valid URL to redirect to".
  Now it says "The URL to redirect to must be on a whitelisted domain"
  for a non-whitelisted redirect, for example.
- Move everything from BulkBase into BulkEditor - BulkEditor is the only
  thing that still used BulkBase.

(Paired with @rgarner on bits of this.)
Amended.