Previously we'd show a Python traceback if the user asked for a detailed
diagnostic in a file that wasn't supported by Clang (something written in Python
perhaps). Now we show an nice, far less scary message informing the user of
this.
Fixes#748.
Previously, we'd implicitly turn off future notices about unknown extra conf
files if we already raised one exception about it. This breaks when the user
ends up not receiving the "unknown extra conf, load?" message.
Now we only turn off the notice as a result of the user saying "don't load this"
so that if the first request fails to reach them, they'll get a second (and
third etc) request about it.
Fixes#615
