Improve error-handling and update Python dependencies

Improve error-handling and update Python dependencies

• Improve error-handling:
  -- Better error messages
• Update Python dependencies:
  -- wheel>=0.35.1
  -- pytest-cov==2.10.1
  -- pytest-xdist==2.0.0;python_version>="3.5"
  -- allure-pytest==2.8.18
• Update examples:
  -- Add test_tinymce.py