Improve failed exemplar messaging

[DataStrategist]

Hi! Me again :)

I'm going to be wrapping the outputs of failed expectations in better prose (so that min(data[["opponent_amount"]], na.rm = TRUE) > 0 is not TRUE becomes opponent_amount should be positive, but isn't or some such.

If you want, I can make these expectations directly in the package and PR it if you're interested?

Also happy to discuss how the errors should be reported:

• opponent_amount should be positive, but isn't
• Expected opponent_amount to be positive
• opponent_amount shouldn't be negative

etc