Briefly introduce dictionaries before using them in the exercise

[tobiashuste]

Make sure that dictionaries are briefly introduced before using them in an exercise.

• The exercise "Reading Error Messages" is moved before the "Syntax Errors" section, in order to ensure learners understood Tracebacks before moving onwards.
• A dictionary is used in the "Reading Error Messages" exercise which has not been introduced before. The learner should be able to get to know dictionaries by this very brief explanation.

Closes #783

[maxim-belkin]

Thanks for the PR, Tobias. We will discuss this PR at our next maintainers meeting.

[maxim-belkin]

Hi Tobias,

Thanks for putting this together. I agree with you and @evezeyl that using dictionaries in an exercise without prior introduction is strange. However, "brief introductions" do increase cognitive load on learners and, therefore, we (maintainers) wonder if some other solution is necessary. The exercise in question does not have to have dictionaries because the key questions it poses are not related to the nature of this data structure. Would you agree to change the exercise to use a list instead of a dictionary? A list can contain names of the days of the week ("Monday", "Tuesday", ..."), first function prints name of the week day that corresponds to the specified day number and the second function calls the first function with index 7.

Thanks for you contribution again! Maxim

CC @swcarpentry/python-novice-inflammation-maintainers