refactor(tvix): Make Store::buildPaths return a Status

Make Store::buildPaths return a Status with [[nodiscard]] rather than
throwing exceptions to signal failure. This is the beginning of a long
road to refactor the entire store API to be status/statusor based
instead of using exceptions.

Change-Id: I2e32371c95a25b87ad129987c217d49c6d6e0c85
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1745
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

third_party/nix/src/nix-daemon/nix-daemon-proto.cc

@@ -21,6 +21,7 @@
 #include "libstore/store-api.hh"
 #include "libutil/archive.hh"
 #include "libutil/hash.hh"
+#include "libutil/proto.hh"
 #include "libutil/serialise.hh"
 #include "libutil/types.hh"
 
@@ -288,9 +289,8 @@ class WorkerServiceImpl final : public WorkerService::Service {
           // TODO(grfn): If mode is repair and not trusted, we need to return an
           // error here (but we can't yet because we don't know anything about
           // trusted users)
-          store_->buildPaths(drvs, mode.value());
-
-          return Status::OK;
+          return nix::util::proto::AbslToGRPCStatus(
+              store_->buildPaths(drvs, mode.value()));
         },
         __FUNCTION__);
   }