Fixing rustdoc stage1.

See #13983 and #14000.

Fix was originally authored by alexcrichton and then rebased a couple
times by pnkfelix, most recently atop PR 13954.

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Regarding the change to librustdoc/lib.rs, to do `map_err` before
unwrapping a `TqskResult`: I do not understand how master is passing
without this change or something like it, since `Box<Any:Send>` does
not implement `Show`.  (Is this something that is only a problem for
the snapshot stage0 compiler?)  Still, the change I have put in here
(which was added as part of a rebase after alex's review) seems
harmless to me to apply to rustdoc at all stages, since a call to
`unwrap` is just going to `fail!` on the err case anyway.

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