Are you talking about using the HTML 5 File API? If so, you should
never run into a FileNotFoundException there if that's one you're
adding since you can't directly open a file, it has to be done with
either a request to the server or through user interaction with a file
dialog or drag and drop. If it's the server that's going to be
throwing that error during an RPC function or something, I think the
client would just get an InvocationException anyways if you're
checking instanceof or something on the exception in the onFailure
method