Stop logging debugmsg when source is a directory

I had the case, where some paths to images are somehow corrupted and only a path to a directory was returned by some other snippet, the phpthumbof filter then tries to make a thumbnail of this directory which results in a huge console spam for each of these images with phpthumb saying it can't create a thumbnail and all the other debug stuff logged, which actually doesn't help a lot, as the source is just not a file...this change fixes this and just logs a skipping message, saying the input source was a directory...