index: Display an error on empty board name

client-data/index.html

 		<div>
 			<p><label for="board">{{{translations.named_private_board_description}}}</label></p>
 			<form action="boards" method="GET">
-				<input type="text" id="board" name="board" placeholder="{{{translations.board_name_placeholder}}}" />
+				<input type="text" id="board" name="board" required placeholder="{{{translations.board_name_placeholder}}}" />
 				<input type="submit" value="Go">
 			</form>
 		</div>