Check res.text as well to return a better error txt message

[AhyoungRyu]

Time to time I see my app throws "Unsuccessful HTTP response" error message when it got unsuccessful response even though a backend sent an error message. But sometimes it throws a correct error that I got from the backend. After some investigation, I realized that sometimes there's no value in res.statusText but only res.text has one. For example, below both responses are from same API request.

Case 1

response: {
  method: 'POST',
  url: 'https://test.url/api/v1/createaccount',
  data: { ... },
  headers: { ... },
  text: 'Unauthorized',
  statusText: '', <-- has no value
  statusCode: 401,
  status: 401,
  statusType: 4,
  info: false,
  ok: false,
  clientError: true,
  serverError: false,
  ...,
}

In this case, it always returns "Unsuccessful HTTP response" which is not a helpful message at all even though backend sent frontend a proper error message in the API response

Case 2

response: {
  method: 'POST',
  url: 'https://test.url/api/v1/createaccount',
  data: { ... },
  headers: { ... },
  text: 'Unauthorized',
  statusText: 'Unauthorized', <-- has the same value with `text` field
  statusCode: 401,
  status: 401,
  statusType: 4,
  info: false,
  ok: false,
  clientError: true,
  serverError: false,
  ...,
}

In this case, it throws me a proper error message as I expected

[niftylettuce]

thanks @AhyoungRyu for your PR, I've published to npm with minor version bump as v5.2.0.

npm install superagent@latest

or with yarn:

yarn add superagent@latest