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
This changes functionality, but since this is an undocumented, non-public API,
it's fine. The reason this is required is because of issue #579; if we try to
run extra conf preload on non-global extra conf, we might not have the
permission to load it. The global extra conf is something the user explicitly
has to set so it's always fine to load that.
