FIX Email notification workflow now ignores recipients with invalid email addresses

[robbieaverill]

Swift mailer in SilverStripe 4 will throw a Swift_RfcComplianceException exception when setTo is called with an invalid email address (like admin).

This change ignores invalid email address formats, rather than letting this exception break the workflow.

Fixes half of #358

[raissanorth]

Seems like a good solution!

[NightJar]

Interesting, I thought SwiftMailer would have a validity check one could leverage rather than relying on catching exceptions... but never the less in light that it doesn't (nor does it particularly matter) this is good :)