[FIX] Partners with invalid emails may stop notification to other partners

Steps to reproduce:

1. Create a partner with an invalid RFC822 email.  For instance, use "-".

2. Create an object (I tested with a task) and add this partner, among others to the followers.

3. Send a message to the followers.

What happens:

OpenERP sends some emails (depends on the ordering of the followers) until it reaches the invalid email.  Then an AssertionError is raised other partners are not notified.


Excepted result:

1. Log the invalid partner but allow notifications to proceeds.  Mark only the message to that partner as not-sent.

Additionally:

1. Avoid to have partners with invalid emails.

Notes:

I have test this in the 7.0 branch (commit 74072441cccb1cadb747e358e1af4a42d1399b86).  Not tested yet in the master or 8.0 branches.