Enhanced HTTP error handling

[serialcool]

Extending HTTP error handling to support headless rendering and reduce configuration efforts.

• enhance the error handling
  • introduce new rest endpoint for error handling
  • move error handling code from servlet to endpoint
  • implement endpoint method for headless rendering
• reduce error handling config
  • convention over configuration
  • fallbackErrorPath --> /
  • relativeErrorPath --> error
  • errorPageNames == errorCode (404, 500, ...)
  • replace existing configuration code