Add expected error code in code blocks flags

[GuillaumeGomez]

Like debated recently with the (not official) doc team and @brson. I'm going to add the possibility to set an expected error code in block code flags for rustdoc check. The main purpose is to avoid that if an error code isn't throw anymore for a same code, it'll be updated in error code explanation as well.

So actually we have for example:

```compile_fail
// the failing code

With this change, it'll be possible to do:

```compile_fail,E0500
// the failing code

And if the E0500 error code isn't thrown at least once, then it'll fail.

It'll be gated of course. I'll start working on it in the next days.

cc @alexcrichton