#69 Improper handling of email errors

[McNolan]

Issue
Certain mail servers will immediately reject invalid recipients with "5.1.1 User unknown". This behaviour is not gracefully handled by filelocker.

Why this is important
We send files to multiple users, and sometimes typos are made. The result causes confusion and is tedious to clean up.

Steps to reproduce
1. Upload a file
2. Create a public share
3. Set the public share to notify 3 users, the second user being an invalid (typo) account at a valid and properly configured mail domain.

Actual results
- User 1 "joe1@mydomain.com" receives a notification
- User 2 "ffff@mydomain.com" fails (Failed to create public share: {'ffff@mydomain.com': (550, '5.1.1 User unknown')})
- User 3 "joe3@mydomain.com" does not receive a notification
- The public share checkbox is now checked
- User history does not contain a "Create Public Share" event

Expected results
1. The first and third emails should be notified of a public share
2. A "Create Public Share" event should be created
3. The user should view a list of emails that were invalid, so they may follow up manually.
4. (Optional) Log valid/invalid email results in the history

[David Hutchins]

Is this still an issue in 2.6?