Defer reporting of build failures in resolver (#9098)
## Summary
In https://github.com/astral-sh/uv/issues/9078, resolution fails because
we fail to build `jsmin`. However... if you look at what's actually
happening, `jsmin` fails to build during _prefetching_. And we never
actually attempt to access its metadata later on.
This PR modifies the metadata result handling such that we don't raise
these errors until the resolver actually asks for the metadata, so
https://github.com/astral-sh/uv/issues/9078 now succeeds.
I actually had to make this change anyway in pursuing
https://github.com/astral-sh/uv/issues/8962, so I've decided to carve it
out here.
Closes https://github.com/astral-sh/uv/issues/9078.
