Make image capture more fault-tolerant

[kentr]

I've been hitting intermittent failures with some paths and running into problems with incomplete directories in the shots folder, so I looked into the error handling a bit and have some questions.

It appears that save_image.rb does have the fallback to using the "invalid image", but it's not working in some cases.

For me, looks like it's because image_was_created() returns true if the mode is resize, even if the image wasn't actually created.

Tracing that, I noticed that attempt_image_capture() is trying to capture images with MULTI_...png in the name, and from looking at _helper.js I gather that the MULTI_...png filename is just a placeholder and not meant to be an actual file.

Is it correct that there aren't supposed to be actual result images named MULTI_...png ?