Find a single HTML element using jQuery-style selectors.

def find_element_by_jquery(browser, selector):
    """Find a single HTML element using jQuery-style selectors."""
    elements = find_elements_by_jquery(browser, selector)
    if not elements:
        raise AssertionError("No matching element found.")
    if len(elements) > 1:
        raise AssertionError("Multiple matching elements found.")
    return elements[0]