Correct HTTP status post for a duplicate plan name

The docstring stated that a POST request would raise a PlanExists but in
practice it didn't and returned a 500. Now the exception is correctly
handled.

Closes-bug: #1359297
Change-Id: Ib62d1f9eafad4e9e7d6764e2105d0750ee0306ab
Merge "Correct HTTP status post for a duplicate plan name"