Add some null checks, and throw descriptive exception when SideTransformer prevents a class from loading.

common/cpw/mods/fml/common/asm/transformers/AccessTransformer.java

@@ -145,6 +145,7 @@ public class AccessTransformer implements IClassTransformer
     @Override
     public byte[] transform(String name, byte[] bytes)
     {
+    	if (bytes == null) { return null; }
         if (!modifiers.containsKey(name)) { return bytes; }
 
         ClassNode classNode = new ClassNode();

common/cpw/mods/fml/common/asm/transformers/MarkerTransformer.java

@@ -89,6 +89,7 @@ public class MarkerTransformer implements IClassTransformer
     @Override
     public byte[] transform(String name, byte[] bytes)
     {
+    	if (bytes == null) { return null; }
         if (!markers.containsKey(name)) { return bytes; }
 
         ClassNode classNode = new ClassNode();

common/cpw/mods/fml/common/asm/transformers/SideTransformer.java

@@ -23,6 +23,8 @@ public class SideTransformer implements IClassTransformer
     @Override
     public byte[] transform(String name, byte[] bytes)
     {
+    	if (bytes == null) { return null; }
+
         ClassNode classNode = new ClassNode();
         ClassReader classReader = new ClassReader(bytes);
         classReader.accept(classNode, 0);
@@ -33,7 +35,7 @@ public class SideTransformer implements IClassTransformer
             {
                 System.out.println(String.format("Attempted to load class %s for invalid side %s", classNode.name, SIDE));
             }
-            return null;
+            throw new RuntimeException(String.format("Attempted to load class %s for invalid side %s", classNode.name, SIDE));
         }
 
         Iterator<FieldNode> fields = classNode.fields.iterator();