rpc: Return precise loadtxoutset error messages

The error messages should never happen in normal operation. However, if
they do, they are helpful to return to the user to debug the issue. For
example, to notice a truncated file.

src/validation.h

@@ -918,7 +918,7 @@ private:
     //! To reduce space the serialization format of the snapshot avoids
     //! duplication of tx hashes. The code takes advantage of the guarantee by
     //! leveldb that keys are lexicographically sorted.
-    [[nodiscard]] bool PopulateAndValidateSnapshot(
+    [[nodiscard]] util::Result<void> PopulateAndValidateSnapshot(
         Chainstate& snapshot_chainstate,
         AutoFile& coins_file,
         const node::SnapshotMetadata& metadata);