Include info on why the error in example might occur

[maneesha]

In the challenge exercise to look for data from 2002 and 2012 there is a message explaining why if (gapminder$year == 2012) results in an error. By itself, this message is confusing. It may be more useful if the message was preceded by an example.

For example, expand the text above:

Did anyone try if (gapminder$year == 2012){print('something')} ` and get a warning message like this?