How to avoid writing to database in form submit?
Form validation is called in the the first part af your code not in the submit function. Add another parameter to your form_alter: 

$form['#validate'][] = 'nameOfMyModule_functionName_validate'; 
$form['#submit'][] = "_validate_entry_webform_client_form_submit_handler";


When validation passes, the submit function will be called. if not form_set_error can be called displaying a message like so:

function nameOfMyModule_functionName_validate($form, &amp;$form_state) {
// Check for the color 'blue'.
    if ($form_state['values']['color'] == 'blue') {
    form_set_error('color', 'Sorry, your favorite color is actually yellow.');
    }
}