Validate JSON input and throw exceptions

Only for the /irrelevant/file and the /explain endpoint, the JSON input is validated. And only these two endpoints throw appropriate exceptions (c.f. !6). These practices should be rolled out to the other endpoints:

• /
• /add
• /remove
• /query
• /state
• /bg
  • POST requests (c.f. #21 (closed))
  • GET requests
• /clear
• /clear/all

Some of these tasks might be done more easy after #27 (closed) is solved.