[Wilds] Allow class loading to continue on failure

Ref T13098. I think I didn't turn this into an actual revision, but let
ClassMapQuery optionally continue if it encounters a class load failure.

If we don't allow this, it can become very difficult to modify or remove
some Arcanist classes since when you, say, remove a Workflow you can no
longer make it to "arc liberate" to update the map for the change.

This is currently used in roughly one place (in an upcoming diff) to let us
get through startup in Runtime and into the "liberate" workflow.