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/application_controller_test.rb

@@ -36,6 +36,45 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
       end
     end
 
+    context "on an unexpected error" do
+      setup do
+        User.stubs(:find).raises(NoMethodError.new("pwned"))
+        @user = create(:user)
+      end
+
+      should "not return the error message in the HTML response" do
+        get user_path(@user)
+
+        assert_response 500
+        assert_match(/NoMethodError/, response.body.to_s)
+        assert_no_match(/pwned/, response.body.to_s)
+      end
+
+      should "not return the error message in the JSON response" do
+        get user_path(@user, format: :json)
+
+        assert_response 500
+        assert_match(/NoMethodError/, response.body.to_s)
+        assert_no_match(/pwned/, response.body.to_s)
+      end
+
+      should "not return the error message in the XML response" do
+        get user_path(@user, format: :xml)
+
+        assert_response 500
+        assert_match(/NoMethodError/, response.body.to_s)
+        assert_no_match(/pwned/, response.body.to_s)
+      end
+
+      should "not return the error message in the JS response" do
+        get user_path(@user, format: :js)
+
+        assert_response 500
+        assert_match(/NoMethodError/, response.body.to_s)
+        assert_no_match(/pwned/, response.body.to_s)
+      end
+    end
+
     context "on api authentication" do
       setup do
         @user = create(:user, password: "password")