Modified the J3M loader to be a little less like a 1980s text adventure.

Added a message to the exception thrown when using an invalid light mode.
Converted it to an IOException instead of the UnsupportedOperationException
(which is a runtime exception) so that the calling code will output a meaningful
error about which asset actually failed.

jme3-core/src/plugins/java/com/jme3/material/plugins/J3MLoader.java

@@ -657,7 +657,7 @@ private void readTechnique(Statement techStat) throws IOException{
                 technique.setLogic(new SinglePassAndImageBasedLightingLogic(technique));
                 break;
             default:
-                throw new UnsupportedOperationException();
+                throw new IOException("Light mode not supported:" + technique.getLightMode());
         }
 
         List<TechniqueDef> techniqueDefs = new ArrayList<>();