Schedule a retry for any alerts that fail

[bx80]

matomo-org/matomo#18335 provides the core scheduler with the ability to retry any scheduled tasks that fail if the task throws a Scheduler\RetryableException.

It would be good to implement this in custom alerts so that any alerts that fail to send can be retried.

[sgiehl]

When implementing this, we need to keep in mind, that the new exception is only available in newer Matomo versions.
I guess the best solution would be to throw that specific exception only if it is available and use a normal Exception if not.
Otherwise we would need to incease the minimum required Matomo version.