[php-slim4] Throw specialized HttpNotImplementedException

Created by: ybelenko

It's a bad practice to use general PHP Exception because it makes difficult to catch specific cases. Slim framework contains Slim\Exception\HttpNotImplementedException class which looks better than unknown Exception.

Result of API call with header Accept: application/json and displayErrorDetails: false option: HTTP/1.1 501 Not Implemented

{
  "message": "501 Not Implemented"
}

and with displayErrorDetails: true option: HTTP/1.1 501 Not Implemented

{
  "message": "501 Not Implemented",
  "exception": [
    {
      "type": "Slim\\Exception\\HttpNotImplementedException",
      "code": 501,
      "message": "How about extending AbstractUserApi by OpenAPIServer\\Api\\UserApi class implementing createUser as a POST method?",
      "file": "/Users/ybelenko/Sites/openapi-generator/samples/server/petstore/php-slim4/lib/SlimRouter.php",
      "line": 1349
    }
  ]
}

If user isn't satisfied with mentioned responses he can:

• Change API controller template or
• Use custom Error Handling Middleware - Slim to catch HttpNotImplementedException and overwrite the result.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon