errors in .ajax are not propagated to .catch

hello,

it seems that errors may not be being properly handled in the ajax operator.

RxJS version:

5.3.0

Code to reproduce:

// index.js (run in node, after installing rxjs, nock, & xmlhttprequest)
var nock = require('nock')
var Observable = require('rxjs').Observable
global.window = global
global.XMLHttpRequest = require('xmlhttprequest').XMLHttpRequest

nock('http://example.com').get('/test/1').reply(200, '!!INVALID-JSON!!') // :eyes:

var obs = Observable
.from([1])
.switchMap(action => {
  return Observable
  .from([2])
  .ajax({ url: 'http://example.com/test/1', method: 'GET' }) // does not work per expectation
  // .map(({ response }) => { throw new Error('bananas') }) // works per expectation
  .catch(err => {
    console.error(err.message) // never reached when JSON wasn't parseable from ajax request
    process.exit()
  })
})

obs.subscribe(arg => console.log('bananas'))

Expected behavior:

.catch entered

Actual behavior:

.catch not entered

Additional information:

Start by running the index.js reproduction against RxJS 5.3.0 and trace the ajax operator's handling of the invalid JSON response. The work is done when the malformed response reaches .catch as an error, matching the expected behavior described in the issue.