Fix error response for API /api/1/get-project.

[five510]

Issue

Resolve https://github.com/ossf/Project-Security-Metrics/issues/47

Changes

• Add Error handling for package_url param.
• Use application/json response in case of status 400 or 404.
• Add auto test for /api/1/get-project
• Change setting and install for test
  • Add testserver in ALLOWED_HOSTS
  • Install django-redis

[five510]

Hi. I'm interested in this project and I made PR for #47 (Not support turning debug mode off). Could you please check it? @scovetta