feat: improve mempool error msg when mempool out of sync (#6618)

Description
---
Improved mempool error message when mempool falls behind the persisted
blockchain db.

Motivation and Context
---
This error will occur in an edge case when the mempool is in the process
of finalizing a block template and just before it is done, a new block
has been added to the blockchain db. As the two processes are
asynchronous, the resolution implemented in the code is to return an
error message so that the client can issue a new request. Meanwhile, the
mempool has time to react to the updated blockchain db state.

How Has This Been Tested?
---
Compile succeed.

What process can a PR reviewer use to test or verify this change?
---
Code review.

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Breaking Changes
---

- [x] None
- [ ] Requires data directory on base node to be deleted
- [ ] Requires hard fork
- [ ] Other - Please specify

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base_layer/core/src/base_node/comms_interface/inbound_handlers.rs

@@ -261,10 +261,15 @@ where B: BlockchainBackend + 'static
                 if last_seen_hash != FixedHash::default() && best_block_header.hash() != &last_seen_hash {
                     warn!(
                         target: LOG_TARGET,
-                        "Mempool out of sync - last seen hash '{}' does not match the tip hash '{}'.",
+                        "Mempool out of sync - last seen hash '{}' does not match the tip hash '{}'. This condition \
+                         should auto correct with the next block template request",
                         last_seen_hash, best_block_header.hash()
                     );
-                    return Err(CommsInterfaceError::InternalError("Mempool out of sync".to_string()));
+                    return Err(CommsInterfaceError::InternalError(
+                        "Mempool out of sync, blockchain db advanced passed current tip in mempool storage; this \
+                         should auto correct with the next block template request"
+                            .to_string(),
+                    ));
                 }
                 let mut header = BlockHeader::from_previous(best_block_header.header());
                 let constants = self.consensus_manager.consensus_constants(header.height);