Formalize HTTP error response format

[mshustov]

Right now error response format repeats Boom interface for BWC

{
  error: string,
  message: string,
  statusCode: string
}

It leads to several problems:

• This contradicts our style guide that requires HTTP response fields to be in shake_case https://github.com/elastic/kibana/blob/master/STYLEGUIDE.md#snake_case
  Changing statusCode --> status_code might affect external consumer relying on the field presence (ElasticCloud for example).
• We don't formalize requirements to the content of the error message, how detailed it should be. We might want to adopt the format of The Problem Details for HTTP APIs RFC that's another attempt for unify format across different platforms.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)