Update ASMHelper.java

Added a helper print statement for locating methods.  I've added this in manually on three occasions now, because I forgot something.
Also added a null-pointer-handler for printOpcodes, so you get an error message where you expect opcodes rather than a complete crash.
Feel free to rejigger as you desire (e.g. boolean flag to turn off the prints in findMethod), this was kind of quick-and-dirty.

Example output for when I forgot to indicate the void return type:
==Patching==
Did you mean '(Lnet/minecraft/world/World;IIILjava/util/Random;)V' instead of '(IIILjava/util/Random;)Lnet/minecraft/world/World;'?
Method was null!
Where did `name` come from? I think you meant `node.name.equals(obf) || node.name.equals(deobf)` here.