Hi Umashiva,
Sorry, if I wasn't clear earlier. What I meant was that you could try to switch to another view. You build a view with an Alert message textbox and a back button to get back to your previous view. As soon as an error occurs, you set a value to a field that triggers a rule that switches view to display the Alert message. You could use a calculated value for this - add a calculated value to the form with some text in it for the warning you wish to display, and conditionally format it to hide if the number field is >= 0.
Browser enabled forms are really restrictive in their functionality. Alerts are not allowed and Microsoft recommends using hidden sections to display messages. i.e. create a hidden section with a text box with message and keep it hidden until you want to display it on some trigger. You can use an expression box which hides if blank, and shows "Alert message" when text is placed into it. Then when you want to display a message, you populate that field, and the expression box displays. Not the best choice in many cases, but it works.
Hope this helps. Thanks!
-Prachi