Image loop fix

This PR addresses a B.O. image managing problem when the source file of an image is missing, mostly because it has been manually deleted. In such case, the image processing fails in the image loop, which not returns the missing image. Thus, the missing image is not displayed in the Image management tab, even if it currently exists in the database.

This is the same problem as #743.

In this PR, the Image loop will return a result even if the image processing fails if at least one of the `backend_context`or `ignore_processing_errors` parameters are true.
Additionally, the loop returns a new variable, `PROCESSING_ERROR`, which is true if a processing error is detected, false otherwise.
Instead of displaying a 'broken image' to the user, the http://placehold.it service is invoked to generate a replacement image : 

![image 488](https://cloud.githubusercontent.com/assets/2197734/5112593/120c614c-7029-11e4-9948-32ba66e99cb9.png)