Update render_doIdentityImages.sh
    
        - Before this commit, if an external file didn't exist, the
          centos-art.sh script just printed a message and continue
          processing the file which provoked inkscape to print an error
          similar to the following:
    
            ---
            ** (inkscape:17154): WARNING **: 
            xlink:href="/home/centos/artwork/trunk/Identity/Themes/Motifs/TreeFlower/Backgrounds/Img/800x600.png">
            did not resolve to a valid image file (base dir is /tmp), now
            trying
            sodipodi:absref="/home/centos/artwork/trunk/Identity/Themes/Motifs/TreeFlower/Backgrounds/Img/800x600.png"
            ---
    
        - In this commit, we solve this issue adding a verification after
          external file checking. If the file is not found, we stop script
          execution and print a message to know more about why the script
          stoped execution.