Improve API data validation in serializers

[mertakozcan]

During create or update process of serializers, sometimes KeyError occurs and currently, we aren't handling them. We can implement validations to check if required fields are present and if there is a problem, we can raise serializers.ValidationError and this returns a message we specify plus meaningful status code.

[kirbiyik]

Quick question @cerob . Should I have a robust validation on the frontend? If we are going to handle bad input it in the backend, I can just show error message which will ease my load since not much left to the milestone.

[cerob]

@kirbiyik That makes your tasks easier and yes, we can do this. :)

[cerob]

I think we can add required attribute so it raises an error during validation.

[cerob]

I am making some of the fields optional according to principles that we have been talking in #99

[kirbiyik]

I think we should go all or none. Otherwise recommendation and search might be troublesome due to gaps in data.

[cerob]

This step is done. There are no KeyErrors anymore. We will be continuing different aspects of validation problems on issue #118