Convert enrollment already exists errors to success.

JIRA: https://openedx.atlassian.net/browse/MA-290

common/djangoapps/enrollment/data.py

@@ -97,7 +97,8 @@ def create_course_enrollment(username, course_id, mode, is_active):
     except CourseFullError as err:
         raise CourseEnrollmentFullError(err.message)
     except AlreadyEnrolledError as err:
-        raise CourseEnrollmentExistsError(err.message)
+        enrollment = get_course_enrollment(username, course_id)
+        raise CourseEnrollmentExistsError(err.message, enrollment)
 
 
 def update_course_enrollment(username, course_id, mode=None, is_active=None):

common/djangoapps/enrollment/errors.py

@@ -30,7 +30,11 @@ class CourseEnrollmentFullError(CourseEnrollmentError):
 
 
 class CourseEnrollmentExistsError(CourseEnrollmentError):
-    pass
+    enrollment = None
+
+    def __init__(self, message, enrollment):
+        super(CourseEnrollmentExistsError, self).__init__(message)
+        self.enrollment = enrollment
 
 
 class CourseModeNotFoundError(CourseEnrollmentError):

common/djangoapps/enrollment/tests/test_views.py

@@ -255,6 +255,11 @@ class EnrollmentTest(ModuleStoreTestCase, APITestCase):
             self.assertTrue(data['is_active'])
         return resp
 
+    def test_enrollment_already_enrolled(self):
+        response = self._create_enrollment()
+        repeat_response = self._create_enrollment()
+        self.assertEqual(json.loads(response.content), json.loads(repeat_response.content))
+
     def test_get_enrollment_with_invalid_key(self):
         resp = self.client.post(
             reverse('courseenrollments'),

common/djangoapps/enrollment/views.py

@@ -17,7 +17,9 @@ from rest_framework.views import APIView
 from opaque_keys.edx.keys import CourseKey
 from opaque_keys import InvalidKeyError
 from enrollment import api
-from enrollment.errors import CourseNotFoundError, CourseEnrollmentError, CourseModeNotFoundError
+from enrollment.errors import (
+    CourseNotFoundError, CourseEnrollmentError, CourseModeNotFoundError, CourseEnrollmentExistsError
+)
 from embargo import api as embargo_api
 from util.authentication import SessionAuthenticationAllowInactiveUser
 from util.disable_rate_limit import can_disable_rate_limit
@@ -339,6 +341,8 @@ class EnrollmentListView(APIView):
                     "message": u"No course '{course_id}' found for enrollment".format(course_id=course_id)
                 }
             )
+        except CourseEnrollmentExistsError as error:
+            return Response(data=error.enrollment)
         except CourseEnrollmentError:
             return Response(
                 status=status.HTTP_400_BAD_REQUEST,