[Wallet sync] Report errors in writing sync metadata

This CL adds missing error reporting for the wallet_metadata sync type.
This brings it on par with how errors are reported for the wallet_data
sync type.

Bug: 1080844
Change-Id: I05b4482c41221636bd53713aed14d1d4b9fe04db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214888
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/master@{#771575}

components/autofill/core/browser/webdata/autofill_wallet_metadata_sync_bridge.cc

@@ -664,7 +664,10 @@ AutofillWalletMetadataSyncBridge::MergeRemoteChanges(
   if (is_any_local_modified) {
     web_data_backend_->NotifyOfMultipleAutofillChanges();
   }
-  return base::nullopt;
+
+  return static_cast<syncer::SyncMetadataStoreChangeList*>(
+             metadata_change_list.get())
+      ->TakeError();
 }
 
 template <class DataType>