Elements are not checked for existence about behatpageobjectextension

[norberttech]

It seems that I also need it, even more than #4 :P
@jakzal what do you think about something like that

abstract class Page
{
    // ... 

    /**
     * @param string $name
     *
     * @return Element
     */
    protected function getElement($name)
    {
        $element = isset($this->elements[$name])
            ? $this->pageFactory->createInlineElement($this->elements[$name])
            : $this->pageFactory->createElement($name);

        if (!$this->has('xpath', $element->getXpath())) {
            throw new UnexpectedPageException(sprintf('"%s" element is not available on the %s', $name, $this->getName()));
        }

        return $element;
    }

   // ...
}

This should be enough, at least for cases that I can imagine.