Bug: ImageManager needs exception handling for non image assets

[JamzTheMan]

ImageManager is attempting to load non-image assets (hero lab statblocks). Need to put some logic around this!

16:26:21.405 (ImageManager.java:318) [pool-6-thread-1] ERROR net.rptools.maptool.util.ImageManager - BackgroundImageLoader.run(D:\Google Drive\Map Tool Resources\Clint\WrathofRighteous\Players\Wrath.por/200/Grayson Darkhammer,6e0926e8e1440b068c3f6a5014a7bf80): image not resolved
java.io.IOException: Could not load image
at net.rptools.lib.image.ImageUtil.bytesToImage(ImageUtil.java:279) ~[bin/:?]
at net.rptools.maptool.util.ImageManager$BackgroundImageLoader.run(ImageManager.java:316) [bin/:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_152]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_152]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_152]