Propagate errors through map

[peterebden]

This is a nicer version of #3150; it should fix the same problem in a more principled way.

Repeating the context here:

• I have subincludes //a and //b, both of which are preloaded.
• //a contains subinclude("//b").
• //b contains a syntax error

We never exit from RegisterPreloads; the errgroup has the error from //b but is waiting for //a to complete, which it never does.

This fixes it by propagating the error through the map where the caller is waiting, so they can receive it too; it extends it from "get the thing or wait" to "get the thing or the error, or wait". I think this should be a more generic solution to this kind of thing overall.