Carry ajax error checking from step #13 forward to other ajax steps

The following steps also have an ajax function, but the 'error:' param
is gone after #13:
#14
#17
#19
#20
This may be superfluous, but it helped me find an error with something I
was doing - Namely, in my .json file, I had single line javascript
comments ("//") that I copied from the tutorial. I couldn't find the
issue on later steps, but was able to see my issue when the error
handler complained about an unexpected "/" in my file in step #13.
[Docs] Rendering null/false and getDOMNode.
Update tutorial.md
Make code spacing more consistent

I'm a little surprised we don't have lint rules for this.
Merge pull request #1386 from georgesisco/master

In the tutorial, carry ajax error checking from step 13 forward to other ajax steps
Merge pull request #1384 from chenglou/render-docs

[Docs] Rendering null/false and getDOMNode
Merge pull request #1403 from spicyj/keyword-spacing

Make code spacing more consistent