Raise Mastodon::HostValidationError when host for HTTP request is private (#6410)

app/lib/exceptions.rb

@@ -4,6 +4,7 @@ module Mastodon
   class Error < StandardError; end
   class NotPermittedError < Error; end
   class ValidationError < Error; end
+  class HostValidationError < ValidationError; end
   class RaceConditionError < Error; end
 
   class UnexpectedResponseError < Error