nix3-build: show all FOD errors with `--keep-going`

Basically I'd expect the same behavior as with `nix-build`, i.e.
with `--keep-going` the hash-mismatch error of each failing
fixed-output derivation is shown.

The approach is derived from `Store::buildPaths` (`entry-point.cc`):
instead of throwing the first build-result, check if there are any build
errors and if so, display all of them and throw after that.

Unfortunately, the BuildResult struct doesn't have an `ErrorInfo`
(there's a FIXME for that at least), so I have to construct my own here.
This is a rather cheap bugfix and I decided against touching too many
parts of libstore for that (also I don't know if that's in line with the
ongoing refactoring work).

Closes https://git.lix.systems/lix-project/lix/issues/302

Change-Id: I378ab984fa271e6808c6897c45e0f070eb4c6fac

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