Is there any easy way to show a warning in the product page and prevent adding product to cart instead of showing the message in cart page, when the quantity increments entered in quantity input field are incorrect?
You can create a block that would be called before_body_end based on events responsible for product listing and product view.
Then create a JS function that will prevent submission of the product view form/product list button unless the min/max/inc quantities are correct in the quantity input. Input quantities would need to be specified with a product-id so you can find associated value in JS array easily.
If you don't understand something or have a problem with specific fragment of code, edit your question and I'll update my answer accordingly.