Unify the exceptions returned in generated Go code

The code generated by the Go compiler is quite awful; every type of exception that can be thrown creates an additional return value, so in the end any call looks like

ret, exA, exB, exC, exD, exE, exF, err = rpc.DoSomething(bla);

Since err is of type os.Error, which should cover all types of exceptions, the attached patch should make the interface significantly more sane by reducing the amount of different return values.