Handle HTTP errors (#15)

* Add HTTP error handling to NetworkQueryer

* Add check to prevent nil pointer dereference

introspection.go

@@ -73,7 +73,7 @@ func IntrospectAPI(queryer Queryer) (*ast.Schema, error) {
 	}
 
 	// if we dont have a name on the response
-	if remoteSchema.QueryType.Name == "" {
+	if remoteSchema == nil || remoteSchema.QueryType.Name == "" {
 		return nil, errors.New("Could not find the root query")
 	}
 

queryer.go

@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"reflect"
+	"strconv"
 
 	"github.com/vektah/gqlparser/v2/ast"
 )
@@ -164,6 +165,11 @@ func (q *NetworkQueryer) sendRequest(acc *http.Request) ([]byte, error) {
 	}
 	defer resp.Body.Close()
 
+	// check for HTTP errors
+	if resp.StatusCode < 200 || resp.StatusCode > 299 {
+		return body, errors.New("response was not successful with status code: " + strconv.Itoa(resp.StatusCode))
+	}
+
 	// we're done
 	return body, err
 }