Delete RuntimeVariableList::from_vec (#7930)

This method is a footgun because it truncates the list. It is the source of a recent bug:

- https://github.com/sigp/lighthouse/pull/7927


  - Delete uses of `RuntimeVariableList::from_vec` and replace them with `::new` which does validation and can fail.
- Propagate errors where possible, unwrap in tests and use `expect` for obviously-safe uses (in `chain_spec.rs`).

beacon_node/store/src/hot_cold_store.rs

@@ -2478,7 +2478,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
                     .first()
                     .map(|blob| self.spec.max_blobs_per_block(blob.epoch()))
                 {
-                    let blobs = BlobSidecarList::from_vec(blobs, max_blobs_per_block as usize);
+                    let blobs = BlobSidecarList::new(blobs, max_blobs_per_block as usize)?;
                     self.block_cache
                         .lock()
                         .put_blobs(*block_root, blobs.clone());