Fixed defexception/1 call

In Elixir 0.13.2 compilation of this module produces the following error:
```
== Compilation error on file lib/decimal.ex ==
** (CompileError) lib/decimal.ex:89: undefined function defexception/1
    (elixir) src/elixir.erl:189: :elixir.quoted_to_erl/3
    lib/decimal.ex:72: (module)
```

That's beacuse from now on `defexception` accepts only two or three parameters, and substitutes the corresponding defmodule semantically (http://elixir-lang.org/docs/stable/Kernel.html#defexception/3). This commit makes `Decimal.Error` compliant with that.