Rollup merge of #101952 - Mark-Simulacrum:missing-fallback, r=ehuss

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.

cc `@ehuss` since IIRC you participated in adding this system

This comes up when building a test nightly from a try build, e.g., https://github.com/rust-lang/rust/pull/101855#issuecomment-1250123298. For now I'm going to manually cherry pick this onto that PR for testing purposes.

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