force_navigate is less grabby

I went through the selenium exceptions, and found a WebDriverError subclass
that's likely good for our needs without supressing other "legitimate"
WebDriver errors, like state elements or element not found.

I also tweaked test generation in test_nav while testing this change, because
the nav destination names make more sense than the string versions of the menu
entries (in my opinion, at least).
Added possibility to use start= in force_navigate correctly

Conflicts:
	cfme/fixtures/pytest_selenium.py
Merge pull request #628 from seandst/force-navigate-eats-less-errors

force_navigate is less grabby