HttpKernel (?) redirects away from error pages, even if they contain verbose fatal errors or exceptions

Problem

• Given:
  • A {sessions} table that has no ssid column.
• When:
  • Trying to log in through User module's standard login block form.
• Then:
  • There is no indication of any error at all; one is redirect to the originating page.

However

• When:
  • Adding the following to the beginning of /core/includes/session.inc:
    var_dump("PREMATURE PAGE OUTPUT");
  • And trying to log in again.
• Then:
  • Suddenly the error appears in the way it is supposed to do:
    Uncaught exception thrown in session handler.

Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'ssid' in 'where clause': SELECT 1 AS expression
FROM
{sessions} sessions
WHERE ( (sid = :db_condition_placeholder_0) AND (ssid = :db_condition_placeholder_1) ); Array
(
    [:db_condition_placeholder_0] => ZVsl9Y4QjdSPtyWDfz2B2Uqs2DTTO0RWhI-wUft8Nsc
    [:db_condition_placeholder_1] =>
)
in Drupal\Core\Database\Connection->query() (line 554 of core\lib\Drupal\Core\Database\Connection.php).

Notes

• Ignore the error condition. — Only the error reporting matters.