Fix "Crop already exists"
Fix "Crop already exists"

As mentioned by @weotch discussed with me in PR #164
"lets say you had a page with the same image with the same crops rendered on it 5 times. On an initial request of this page, the crop would not exist. Lets say there were 5 http server threads handling these requests. If the requests all hit at the same time, all 5 would try to generate the image and save it to disk. One of them would finish first and then the other 4 would throw Crop already exists exceptions. I feel like this is a valid use case and IS a reason that I should make this condition not fatally error. It should just silently return the image response.

However, I think the fix should be a change to writeCrop where it just returns rather than throwing an Exception."
BKWLD/croppa,BKWLD/croppa