Add client/server error handling logic to template

[psolymos]

As described in #1 we can add try/catch mechanism to distinguish client and server errors.

The main differences:

• client error:
  • should have HTTP status 4xx
  • indicates a user error that we cannot fix, but we can instruct the user
  • 4xx error message can be used by front end to inform user about the error
• server error
  • this is the default 500 error message that Plumber is using via its error handing hook
  • indicates a server error that hasn't surfaced yet, and admins/devs need to fix it
  • user should not see the details, only a generic message
  • admins/devs should be able to find the error message to see what failed

The PR adds mariobox::http_error() as a drop in replacement for stop() that contains the message and the intended status code.

mariobox::mario_try() wraps the internal function and modifies the response object based on the class of the condition object when an error happened.

[ALanguillaume]

Thanks a lot for the PR ! Sorry it was left on hold for so long. We had very little bandwidth to work on the project.