Improve API error handling

[SlowMo24]

While using the API I experienced the following inconveniences that might be worth improvement:

• the API response with an HTML page on faulty requests. Although that is ok-ish, it means I have to write the response to a file and open that in my browser to get the error message. Responding in a more common manner with the correct HTTP status code and a json response may be preferable. The response could still contain a link to the error page for humans, if needed.
• when sending a faulty POST request to the /incidents endpoint (e.g. with an invalid data like below), I get redirected to root and the final error only tells me that POST is not allowed for / which is confusing:

example invalid data that will spawn the redirection to root:

{
  "name": "<string>",
  "status": 0,
  "message": "<string>",
  "template": "<string>",
  "visible": false,
  "stickied": false,
  "notifications": false,
  "occurred_at": "<string>",
  "template_vars": [],
  "component_id": 123,
  "component_status": 1
}

• when omitting the API token from the POST request, a redirect failure happens instead of a https://en.wikipedia.org/wiki/HTTP_403 :

[jbrooksuk]

What headers are you sending when making the API requests? You should only need to add:

Accept: application/json
Content-Type: application/json