Failed email notifications are not retried

This issue has been migrated from [#2325](https://github.com/matrix-org/synapse/issues/2325).

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When there is an exception "Exception processing notifs" ([emailpusher.py line 134](https://github.com/matrix-org/synapse/blob/master/synapse/push/emailpusher.py#L134)) while sending e-mail push notifications, sending that e-mail failed.

Such e-mail push notifications are not scheduled to be re-tried later, means they are basically treated as if they succeeded. In particular, the timers to delay further e-mails are also started (see top of [emailpusher.py](https://github.com/matrix-org/synapse/blob/master/synapse/push/emailpusher.py) for the constants that define them).

This makes testing e-mail notifications very difficult, as usually no further e-mails will be sent for hours after one e-mail push notification was processed, successfully or not.

So may I propose to re-schedule unsuccessful e-mail push notifications, without starting the e-mail delaying timers.