Provide better error message if a Page was not added to the PageObjectMap

[thompsnm]

Currently, if a user tries to call a step like

Given I am on the "Foo" page

without adding a page object named 'Foo' to the PageObjectMap, he gets an error message that reads:

TypeError: undefined is not a function

Given that this mistake is fairly common, the error message should be much more actionable. Something like:

Could not find page with name 'Foo' in the PageObjectMap, did you remember to add it?