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/libstore/store-api.hh

@@ -445,7 +445,8 @@ class Store : public std::enable_shared_from_this<Store>, public Config {
      output paths can be created by running the builder, after
      recursively building any sub-derivations. For inputs that are
      not derivations, substitute them. */
-  virtual void buildPaths(const PathSet& paths, BuildMode buildMode = bmNormal);
+  [[nodiscard]] virtual absl::Status buildPaths(const PathSet& paths,
+                                                BuildMode buildMode = bmNormal);
 
   /* Build a single non-materialized derivation (i.e. not from an
      on-disk .drv file). Note that ‘drvPath’ is only used for