Exceptions and catchable errors in Mailer::sendMail cause incorrect statistics

Problem/Motivation

During spooling (for example via cron, drush or UI), Mailer::sendSpool keeps track of the sent emails, but doesn't store it until the very end of the process. This means if anything interrupts that process, for example an exception, causes it to lose track of all the emails stats it had previously built up.

In my specific case, my error was:
TypeError: Argument 1 passed to Drupal\Core\Session\AccountSwitcher::switchTo() must implement interface Drupal\Core\Session\AccountInterface, null given, called in /var/www/html/web/modules/contrib/simplenews/src/Mail/MailEntity.php on line 256 in Drupal\Core\Session\AccountSwitcher->switchTo() (line 58 of /var/www/html/web/core/lib/Drupal/Core/Session/AccountSwitcher.php).

Proposed resolution

Mailer::sendMail should be as aggressive as possible at catching errors in the sending process without falling over. I would suggest that it has a catch (\Throwable $throwable) { ... } to handle as many possible scenarios.

It seems a reasonable assumption that it will have failed to send, so should be tracked as skipped, and should be treated as an error. It would also make sense to log these errors so that they're not simply silenced.

Remaining tasks

Do it!

User interface changes

None.

API changes

Fewer unhandled errors causing problems.

Data model changes

None.

Release notes snippet

Mailer::sendMail has been made more resilient to errors in the mail building and sending process by catching all throwable errors and exceptions, logging them and treating the email as a permanent failure.