Return optional error from ApplyArgsManOptions

Also pass in a (for now unused) reference to the params.

Both changes are needed for the next commit.

src/test/util/setup_common.cpp

@@ -160,7 +160,8 @@ CTxMemPool::Options MemPoolOptionsForTest(const NodeContext& node)
         // chainparams.DefaultConsistencyChecks for tests
         .check_ratio = 1,
     };
-    ApplyArgsManOptions(*node.args, mempool_opts);
+    const auto err{ApplyArgsManOptions(*node.args, ::Params(), mempool_opts)};
+    Assert(!err);
     return mempool_opts;
 }