404's on API requests for slots that have been skipped or orphaned (#2272)

## Issue Addressed

Resolves #2186

## Proposed Changes

404 for any block-related information on a slot that was skipped or orphaned

Affected endpoints:
- `/eth/v1/beacon/blocks/{block_id}`
- `/eth/v1/beacon/blocks/{block_id}/root`
- `/eth/v1/beacon/blocks/{block_id}/attestations`
- `/eth/v1/beacon/headers/{block_id}`

## Additional Info



Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/http_api/src/block_id.rs

@@ -60,6 +60,27 @@ impl BlockId {
             CoreBlockId::Head => chain
                 .head_beacon_block()
                 .map_err(warp_utils::reject::beacon_chain_error),
+            CoreBlockId::Slot(slot) => {
+                let root = self.root(chain)?;
+                chain
+                    .get_block(&root)
+                    .map_err(warp_utils::reject::beacon_chain_error)
+                    .and_then(|block_opt| match block_opt {
+                        Some(block) => {
+                            if block.slot() != *slot {
+                                return Err(warp_utils::reject::custom_not_found(format!(
+                                    "slot {} was skipped",
+                                    slot
+                                )));
+                            }
+                            Ok(block)
+                        }
+                        None => Err(warp_utils::reject::custom_not_found(format!(
+                            "beacon block with root {}",
+                            root
+                        ))),
+                    })
+            }
             _ => {
                 let root = self.root(chain)?;
                 chain