Provide a solution for the first part of exercise 4

[NoamBechhofer]

In chapter 4, the first part of the exercise asks the reader to explain why an error occurs, however there is no solution to this part of the exercise. Therefore the reader may be unsure whether their answer is correct. This can be remedied by either adding a click-and-reveal solution/answer, or including the answer in the solutions subdirectory

[szybkigustaw]

That would be a great idea.
More interesting is the fact that when I created a separate rust bin project and copy-pasted the code from the section in question, it compiled and ran without any errors, which makes the whole exercise even more confusing.