Handle RestClient errors in Discord integration

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2441928/?referrer=gitlab_plugin

RestClient::NotFound: 404 Not Found
(163 additional frame(s) were not displayed)
...
  net/http.rb:933:in `start'
    return yield(self)
  restclient/request.rb:743:in `block in transmit'
    process_result(res, start_time, tempfile, &block)
  restclient/request.rb:836:in `process_result'
    response.return!(&block)
  restclient/abstract_response.rb:129:in `return!'
    raise exception_with_response
  restclient/abstract_response.rb:249:in `exception_with_response'
    raise klass.new(self, code)

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2450081/?referrer=gitlab_plugin

RestClient::BadRequest: 400 Bad Request
(163 additional frame(s) were not displayed)
...
  net/http.rb:933:in `start'
    return yield(self)
  restclient/request.rb:743:in `block in transmit'
    process_result(res, start_time, tempfile, &block)
  restclient/request.rb:836:in `process_result'
    response.return!(&block)
  restclient/abstract_response.rb:129:in `return!'
    raise exception_with_response
  restclient/abstract_response.rb:249:in `exception_with_response'
    raise klass.new(self, code)

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2465702/?referrer=gitlab_plugin

RestClient::BadGateway: 502 Bad Gateway
(163 additional frame(s) were not displayed)
...
  net/http.rb:933:in `start'
    return yield(self)
  restclient/request.rb:743:in `block in transmit'
    process_result(res, start_time, tempfile, &block)
  restclient/request.rb:836:in `process_result'
    response.return!(&block)
  restclient/abstract_response.rb:129:in `return!'
    raise exception_with_response
  restclient/abstract_response.rb:249:in `exception_with_response'
    raise klass.new(self, code)