Extended error message not available for privacy errors

[jethrogb]

On currently nightly, I get:

$rustc --explain E0446
error: no extended information for E0446

(playpen)

Yet the description for this error was merged weeks ago: #28312

[Manishearth]

cc @GuillaumeGomez , in the future when adding new diagnostics mods be sure to add a build_diagnostic_array call