Improve error handling of synchronous notifications

[ssddanbrown]

Notification email sending failures, without using a queue/worker currently throw and block the process, showing the error view.
Since email requires the network, and SMTP servers can respond in various unexpected ways, and since notifications sending issues probably wouldn't be considered an essential error to the majority of cases, would should probably instead attempt these but catch & log errors, to allow the action within BookStack to continue as expected.

Related to support thread 125

Doc Updates

Probably worth documenting error behaviour on the docs page.