reduce boilerplate in handling fetch responses and errors in redux actions

fetchUtils.js

function isJson (response) {
  const contentType = response.headers.get('content-type')
  return contentType && contentType.indexOf('application/json') !== -1
}

export function handleResponse (response) {
  if (response.ok) {
    if (isJson(response)) {
      return response.json()
    } else {
      return response.text()
    }
  } else {
    const error: ErrorT = new Error(response.statusText)
    error.response = response
    throw error
  }
}

export function handleError(reject, dispatch, errAction) {
  return function(err) {
    if (err.response && isJson(err.response)) {
      err.response.json()
        .then((errBody) => {
          err.body = errBody
          reject(err)
          dispatch(errAction(err))
        })
    } else {
      reject(err)
      dispatch(errAction(err))
    }
  }
}