The file to check is /include/func/func.cart.php
Code:
function func_cart_check_maxmin_limits_n_redirect($cart)
In our latest reBOOT template, we put a static alert message in the cart, so as a workaround you could do the same...
DEMO
In /skin/reboot/customer/main/cart.tpl
Add this code wherever you like...
Code:
{if $config.General.minimal_order_amount ne 0 && $cart.display_subtotal lt $config.General.minimal_order_amount}
<div class="alert alert-danger">
{assign var="symbol" value="$"}
{assign var="minimal_order_amount" value="<b>`$symbol``$config.General.minimal_order_amount|string_format:"%.2f"`</b>"}
{$lng.err_checkout_not_allowed_msg|substitute:"value":$minimal_order_amount}
</div>
{/if}