Explain differences between mean error, mean squared error and root mean squared error in Chapter 8

[rjoberon]

In Chapter 8 three error measures are presented and used: mean error, mean squared error (MSE), and root mean squared error (RMSE). Students often ask, what the difference between those measures is and when (and why) which measure is used. It would be helpful to have short explanations and justifications within that chapter.

[AllenDowney]

Thanks for passing this along -- it is always useful to know where readers get confused or have more questions. I will look for a good way to explain this.