univalue: Return more detailed type check error messages

test/functional/rpc_help.py

@@ -92,7 +92,7 @@ class HelpRpcTest(BitcoinTestFramework):
         assert_raises_rpc_error(-1, 'help', node.help, 'foo', 'bar')
 
         # invalid argument
-        assert_raises_rpc_error(-1, 'JSON value is not a string as expected', node.help, 0)
+        assert_raises_rpc_error(-1, "JSON value of type number is not of expected type string", node.help, 0)
 
         # help of unknown command
         assert_equal(node.help('foo'), 'help: unknown command: foo')