In the tutorial, 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 step 13:
step 14
step 17
step 19
step 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 step 13.