BigPipe causes PHP notices with JavaScript disabled on error pages

Problem/Motivation

Under some circumstances, BigPipe throws PHP notices because it tries to explode on <drupal-big-pipe-scripts-bottom-marker> in \Drupal\big_pipe\Render\BigPipe::sendPreBody(), while that marker is missing.

This seems to happen when subrequests are created via DefaultExceptionHtmlSubscriber with javascript disabled.

Notices:

Notice: Undefined offset: 1 in Drupal\big_pipe\Render\BigPipe->sendPreBody() (line 143 of core/modules/big_pipe/src/Render/BigPipe.php).
Notice: Undefined offset: 2 in Drupal\big_pipe\Render\BigPipe->sendPreBody() (line 143 of d8/core/modules/big_pipe/src/Render/BigPipe.php).

Steps to reproduce:

On a fresh D8.1.x, create some content, disable javascript in your browser, visit /page-does-not-exist: this should trigger the above notices in the log.