Avoid panicking on missing fallback

This just prints a message but continues on if a fallback is missing,
which can happen when we're building a partial set of builders and
producing a dev-static build from it (e.g., when no Apple builder runs
at all).

Probably the more extensive fix is to allow the build-manifest invoker
to specify the expected set of targets & hosts, but that's a far more
extensive change. The main risk from this is that we accidentally start
falling back to linux docs across all platforms without noticing. I'm
not sure that we can do much about that though at this time.

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