use explicit error responses in tests

[dwickern]

Previously these tests simulate an error response with pretender's catchall unhandled request:

Error: Pretender intercepted PUT /people/1 but no handler was defined for this type of request
    at Pretender.unhandledRequest (pretender.es.js:297:1595)
    at Pretender.handleRequest (pretender.es.js:297:263)
    at FakeRequest.send (pretender.es.js:284:496)
    at eval (pretender.es.js:264:85)
    at new Promise (<anonymous>)
    at fetch (pretender.es.js:263:1705)
    at ApplicationAdapter._fetchRequest (rest.js:943:1)
    at ApplicationAdapter.ajax (rest.js:908:1)
    at ApplicationAdapter.updateRecord (rest.js:701:1)
    at -private.js:1022:1

This is annoying because it trips the debugger's uncaught exception breakpoint.