provide a better missing error message

When github say's missing, it's not talking about the resource - it
means something referenced in the data.

For example, specifying a milestone that doesn't exist will return:

```
{
	"message": "Validation Failed"
	"documentation_url": "http"://developer.github.com/v3"
	"errors": [
		{
			"value": 42,
			"resource": "Issue",
			"field": "milestone",
			"code": "missing"
		}
	]
}
```

Previously this would be reported as:

> Validation Failed: Resource "Issue" not exists anymore

Now it's reported as

> The milestone 42 doesa not exist, for resource "Issue"

Author: AD7six

lib/Github/HttpClient/Listener/ErrorListener.php

@@ -57,7 +57,7 @@ public function postSend(RequestInterface $request, MessageInterface $response)
                     foreach ($content['errors'] as $error) {
                         switch ($error['code']) {
                             case 'missing':
-                                $errors[] = sprintf('Resource "%s" not exists anymore', $error['resource']);
+                                $errors[] = sprintf('The %s %s does not exist, for resource "%s"', $error['field'], $error['value'], $error['resource']);
                                 break;
 
                             case 'missing_field':

test/Github/Tests/HttpClient/Listener/ErrorListenerTest.php

@@ -124,6 +124,7 @@ public function shouldNotPassWhen422IsSentWithErrorCode($errorCode)
                 array(
                     'code'     => $errorCode,
                     'field'    => 'test',
+                    'value'    => 'wrong',
                     'resource' => 'fake'
                 )
             )