Add validation that POST requests are in the correct format

[MLHProgram]

When making a POST request to add an Experience, Education or Skill, the API server should check it's in the correct format, including all the required fields. If it is not, it should return an error message and the appropriate HTTP status code.

This should also include writing a test inside of test_pytest.py as part of the feature to show that it works. If you're unfamiliar with Pytest, you should take a read inside the LMS for more instructions!