Add video on how to investigate errors, including creating a reprex

[mpjashby]

In particular, show the iterative process of writing a bit of code, checking the output, fixing any errors and then writing the next bit.

[mpjashby]

Changed this to Week 3 to fit with the tutorial on managing errors.

[mpjashby]

There was not time for this this year, so it will have to wait for next year.

[mpjashby]

https://youtu.be/hlcftgeTAyY