Update 17.markdown - Errors are structs.

Changed the documentation in getting_started/17 to reflect that Exceptions are structs.  I'm still learning the language, so if I'm wrong on this bear with me :smile: 

**Old Example:**

```iex
iex> try do
...>   raise "oops"
...> rescue
...>   e in RuntimeError -> e
...> end
RuntimeError[message: "oops"]
```

**Local Result:**
```iex
iex> try do
...>   raise "oops"
...> rescue
...>   e in RuntimeError -> e
...> end
%RuntimeError{message: "oops"}
```