Handle invalid URLs on iOS, instead of crashing
Handle invalid URLs on iOS, instead of crashing

Before, when loading a non-image (e.g. a 404 html page), the code/app would crash because function `contentTypeForImageData` would return `nil` as the file extension, which was not accepted by `NSURL.URLByAppendingPathExtension`.
Now, the spinner will disappear, and a simple alert will be shown.
sarriaroman/photoviewer,sarriaroman/photoviewer,sarriaroman/photoviewer