Don't show error messages that could contain private information.

Fix a potential exploit where private information could be leaked if
it was contained in the error message of an unexpected exception.

For example, NoMethodError contains a raw dump of the object in the
error message, which could leak private user data if you could force a
User object to raise a NoMethodError.

Fix the error page to only show known-safe error messages from expected
exceptions, not unknown error messages from unexpected exceptions.

API changes:

* JSON errors now have a `message` param. The message will be blank for unknown exceptions.
* XML errors have a new format. This is a breaking change. They now look like this:

    <result>
      <success type="boolean">false</success>
      <error>PaginationExtension::PaginationError</error>
      <message>You cannot go beyond page 5000.</message>
      <backtrace type="array">
        <backtrace>app/logical/pagination_extension.rb:54:in `paginate'</backtrace>
        <backtrace>app/models/application_record.rb:17:in `paginate'</backtrace>
        <backtrace>app/logical/post_query_builder.rb:529:in `paginated_posts'</backtrace>
        <backtrace>app/logical/post_sets/post.rb:95:in `posts'</backtrace>
        <backtrace>app/controllers/posts_controller.rb:22:in `index'</backtrace>
      </backtrace>
    </result>

  instead of like this:

    <result success="false">You cannot go beyond page 5000.</result>

test/functional/static_controller_test.rb

@@ -68,7 +68,7 @@ class StaticControllerTest < ActionDispatch::IntegrationTest
       get "/qwoiqogieqg", as: :xml
 
       assert_response 404
-      assert_equal("Page not found", response.parsed_body.at("result").text)
+      assert_equal("Page not found", response.parsed_body.xpath("result/message").text)
     end
 
     should "render the 404 page when page_not_found_pool_id is configured" do