attach onerror callback to the image used for ajax#getImage

[kturney]

I ran into there being no error callback while trying to map#loadImage a url to an svg instead of a png.

The error message could probably be better, and I'm not sure how y'all feel about attaching the original error as the domErr property.

The getImage, invalid image test is currently failing with

Error: Fake server request processing threw exception: URL.createObjectURL is not a function
      at TypeError: URL.createObjectURL is not a function
      at     at exports.getArrayBuffer (src/util/ajax.js:136:53)
      at     at FakeXMLHttpRequest.xhr.onload (src/util/ajax.js:94:13)
      at     at Array.forEach (<anonymous>)
      at     at Object.window.server.respondWith.request [as response] (test/unit/util/ajax.test.js:100:21)
      at     at Array.forEach (<anonymous>)
      at     at Test.t.test (test/unit/util/ajax.test.js:107:23)

so I'm not sure why that wouldn't be there.

[jfirebaugh]

We use jsdom in the tests, and it doesn't implement URL.createObjectURL or URL.revokeObjectURL. Unless there's some creative way to bypass those with mocks, we might not be able to test this currently.

[kturney]

So what is the desired solution?

Delete the tests or (somewhat hackily) stub out URL.createObjectURL and URL.revokeObjectURL if they do not exist?

I can implement whichever you prefer.