Fix bug: error messages are not shown in Course Team Management

Description

*Backround:* This PR fixes a bug in Instructor Dashboard > Membership > Course Team Management. Error messages were not shown when attempted to assign a role to a non-existing or inactive user. See screenshots 'Before' and 'After'.

*Studio updates:* None

*LMS updates:* proper template message call in `lms/static/js/instructor_dashboard/membership.js`
A unit test has been added to check the fix (`lms/static/js/spec/instructor_dashboard/membership_auth_spec.js`).

*Test plan:*
Before the fix:
In lms, sign in as staff. Go to Instructor > Membership > Course Team Management.
Try to add a non-existing user as 'staff' member.
The user is not added, an error message is NOT shown. An error is thrown to the browser’s js console.
The same scenario occurs if you try to assign 'staff' role to an inactive user.
After the fix:
Try to add a non-existing user as 'staff' member.
An error message IS shown.
Try to add an inactive user as 'staff'.
An error message is shown as well.

![Before_NonexistingUser](https://user-images.githubusercontent.com/33125830/54290101-0a8d3680-45b3-11e9-9817-592b12519281.png)
![After_NonexistingUser](https://user-images.githubusercontent.com/33125830/54290124-137e0800-45b3-11e9-9fb3-b271ded060c2.png)

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Contributor Name

Iaroslavna Vasylieva

Repo

edx/edx-platform

Customer

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

None

Platform Map Area (Levels 3 & 4)

None

Blended Hour Utilization Percentage

None

Priority

Unset
Configure