code: function readSingleWallet(conn, handleWallet){
  	conn.query("SELECT wallet FROM wallets", function(rows){
  		if (rows.length === 0)
  			throw Error("no wallets");
  		if (rows.length > 1)
  			throw Error("more than 1 wallet");
  		handleWallet(rows[0].wallet);
  	});
  }
docs: read single wallet, If amount of wallets is bigger than one, will throw an error
  @param {object} conn - database connection
  @param {function} handleWallet - callback