When ccassignee is not an Assignable user, no error is logged

When sending mail to JIRA with ccassignee on, either the CC field or the second recipient (in the TO field) is made the assignee. If this user does not have assignable features, something else will happen, but this is not logged. For scenarios where there are multiple users with the same email address and one is assignable and one is not, we need the error to specify the user_name attribute for comparison.

Steps to reproduce:

1. Install JIRA
2. Create two users with the same email address
3. Make one user assignable
4. Configure handler/mail server
5. Send email to JIRA cc assignee
6. Confirm that no warning is in log
7. Confirm that the created ticket is not assigned to CC