Change Alphabet::EncodeSingle and Alphabet::DecodeSingle to use an outparam and return an error code instead of calling `abort`

I run into the error "Invalid label 0". I assume it is from the function Alphabet::DecodeSingle() in the file native_client/alphabet.cc.

I don't know why I am running into this error and it is not important for now. What is important is the fact that the code calls abort() just after printing this error.

I don't know your guidelines. But I can tell that aborting the whole process, including the python interpreter which might be interactive, is a terrible way of handling errors, especially in such a trivial function, and I call it a bug.