Error page layout broken due to leftovers from template output buffer

Problem/Motivation

If _drupal_log_error() hits within a template, output buffer contents are printed before the maintenance page, causing broken layout.

Steps to reproduce

An easy "hack" to artificially reproduce this is to insert the following code inside theme():
(other places might also work)

function theme(..) {
  [..]
  static $thrown = true;
  // Capture $ob_contents in a variable for debugging.
  if (!$thrown && $ob_contents = ob_get_contents()) {
    $thrown = TRUE;
    throw new \Error('Fake error in theme().');
  }
  [..]

(Typically such errors would happen in arbitrary places due to programming errors, but it is simpler to generate it artificially.)

When visiting the site with a browser, the html source will contain the output buffer contents + the maintenance page, causing a broken layout.

Technical explanations

A demonstration of the effect with plain php: https://3v4l.org/6r9b4

function exception_handler($e) {
    print "Exception page\n";
    exit();
}

set_exception_handler('exception_handler');

ob_start();
print "A\n";
ob_start();
print "B\n";

throw new Exception();

Special considerations

• 'Throwable' and 'Error' exist since PHP 7.0.0.
  Since then, fatal errors behave similarly to exceptions and go to exception handler instead of the error handler.
• 'finally' is supported since PHP 5.5.0. We could use it, but I think we don't need to.
• Initial output buffer level may be 1 or 0 depending on the 'output_buffering' setting in php.ini, see https://www.php.net/manual/en/outcontrol.configuration.php. It seems Drupal works fine with either value.
• Drupal starts an output buffer level with ob_start() in _drupal_bootstrap_page_header(), and ends it in drupal_page_footer().

Proposed resolution

Different options:

• Clear / end all output buffer levels, before printing the maintenance page, in _drupal_log_error().
  This would also close the output buffer from _drupal_bootstrap_page_header(), and the one from php itself. Not sure if that's good or bad.
• Use try / catch in theme_render_template(), to always end the output buffer, even when a fatal error or exception occurs.
  This should be a safe improvement, and sufficient for the case discussed here.