When !keepGoing and a goal fails, other goals are cancelled and
remain with exitCode == ecBusy. These cancelled goals have a default
BuildResult::Failure{} with empty errorMsg.
Previously, buildPathsWithResults would return these cancelled goals,
and throwBuildErrors would report them as failures. When only one such
cancelled goal was present, it would throw an error with an empty
message like:
error: build of '/nix/store/...drv^*' failed:
Now we skip goals with ecBusy since their state is indeterminate.
Cancelled goals could be reported, but this keeps the output relevant.
Other indeterminate goal states were already not being reported, for
instance: derivations that weren't started for being blocked on a
concurrency limit, or blocked on a currently building dependency.
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