feat[plugin,webhook]: Add support for custom error responses

[satterly]

Example

    def take_action(self, alert, action, text, **kwargs):
        if 'action' != 'tea':
            raise AlertaException("I'm a teapot", code=418,
                                  errors=[
                                      'server refuses to brew coffee because it is, permanently, a teapot',
                                      'See https://tools.ietf.org/html/rfc2324'
                                  ])
        return alert, action, text

Output of the above

{
  "code": 418,
  "errors": [
    "server refuses to brew coffee because it is, permanently, a teapot",
    "See https://tools.ietf.org/html/rfc2324"
  ],
  "message":"I\'m a teapot",
  "requestId": "a9d963ba-5ec0-40a4-88a6-84c33381384a",
  "status": "error"
}

References
https://tools.ietf.org/html/rfc2324