Log ImageGenerator requirements

[rrmesquita]

I was taking a look at the ImageGenerator class trying to understand why my registerMediaConversions wasn't being called at all during a file upload. As expected, the package verifies if the required tools are installed (in my case, ImageMagick and GhostScript, for generating PDF thumbnails).

My surprise is that this goes silent. No exception or log is generated when it fails. I assume that this is because the way this method was coded, but still, I wish this was more explicit.