output better error when trying to create already existing cohort

[ilevantis]

currently looks like this:

> cb_create_cohort("ilya-r-test1", cb_version="v1")
Error in .cb_create_cohort_v1(cohort_name = cohort_name, cohort_desc = cohort_desc,  : 
  Bad Request (HTTP 400). Failed to create a cohort.
> cb_create_cohort("ilya-r-test2", cb_version="v2")
Error in .cb_create_cohort_v2(cohort_name = cohort_name, cohort_desc = cohort_desc,  : 
  Conflict (HTTP 409). Failed to create a cohort.

[ilevantis]

response does contain more useful information.
find it at res$message

  r <- httr::POST(url,
                  .get_httr_headers(cloudos$token),
                  query = list("teamId" = cloudos$team_id),
                  body = list("name" = cohort_name,
                              "description" = cohort_desc, 
                              "moreFilters" = filters),
                  encode = "json"
  )
  if (httr::http_error(r)){
    message(httr::content(r)$message)
    httr::stop_for_status(r, task = "create a cohort")
  }
  # parse the content
  message("Cohort created successfully.")
  res <- httr::content(r)

http_error(x, ...)