SUP-1813 Attempting to update relationship to invalid type freezes dialog.

Replicated on dmaster.demo.civicrm.org 10 Jul 2017

Agileware reference SUP-1813

Steps to reproduce (demo system):

1. Change the "Volunteer is" relationship contact type A to "Individual - Volunteer"
2. Assign an non-Volunteer Individual to be an Employee of an organisation.
3. From the Organisation's contact, edit the Relationship
4. Change the type to "Volunteer is"
5. Press Save

Expected outcome

Some kind of error message.

Actual outcome

• Spinning triangles type freeze.
• Inspecting the XHR response reveals that a marked up error page is produced instead of a JSON snippet

Proposed solution

For preference, this relationship type should not even be available from the drop-down. See CRM-16741 for a related scenario that's allegedly been solved.

Failure should at least show the "Invalid Relationship" message.