Server Error (500) when one add an existing member to a team

Description

The system has an Internal Server Error (HTTP 500) when a logged user that owns a team tries to add a member with an email that have already been added before. This happens due to an IntegrityError on database.

Steps to Reproduce

• Given I am logged in as the user with email 'paul@example.com'
• And I own the team "My Team"
• When I go to the manage members for team "My Team" (/teams/my-team/+manage/)
• I should see my email on the Team Members list
• When I add my own email in the 'Add user to team' textfield
• And I press the 'Add member' button
• The system has an Internal Server Error returning a HTTP 500 error code

Expected behavior

1. Identify the error in JoinView
2. Redirect to the add member page (dtracker-team-join)
3. Show an alert saying that it was not possible to add the user because his/her already belongs to the team.