Allow to retrieve last error

[fritzmg]

Currently when using oneup/mailchimp-api-v3 it is not really possible to receive detailed error messages from Mailmhimp. For example, if you use the subscribeToList method, Mailmhimp might response with something like this in its response:

{
  "type": "https://mailchimp.com/developer/marketing/docs/errors/",
  "title": "Invalid Resource",
  "status": 400,
  "detail": "Your merge fields were invalid.",
  "instance": "8cf2c011-a212-9b3a-6c51-a4d631a124b4",
  "errors": [
    {
      "field": "VORNAME",
      "message": "Please enter a value"
    },
    {
      "field": "NACHNAME",
      "message": "Please enter a value"
    }
  ]
}

i.e. telling you that the merge fields you submitted are wrong and which ones and why.

This PR would always save the JSON encoded body of the non-200 responses as the last error so that you can retrieve it from the outside.

[bytehead]

I like!

[bytehead]

There you go: 2.4.0