Long error diagnostics should not show compiletest/highlighting metadata in text mode

e.g. 

``````
$ rustc --explain E0413
A declaration shadows an enum variant or unit-like struct in scope. Example of
erroneous code:

```compile_fail
struct Foo;
...
``````

Stuff like ````compile_fail` shouldn't show, just the backticks.

cc @GuillaumeGomez @steveklabnik 
