Improving erro handling while trying to reserve tasks at async runner

Each time when MQ becomes unreachable HTTP GET /tasks returned HTTP 500
 and code was not handling this case except expecting networking errors.
 After that it tried to unmarshal empty response body that caused another sort of an error.

 This patch triggers error based on http response code, explicitly checking if response code
 is something unexpected (not HTTP 200 OK).

 Response status code for /tasks for changed from 202 Accepted to 200 OK according to swagger doc.

api/runner/async_runner.go

@@ -14,30 +14,37 @@ import (
 	"sync"
 	"time"
 
+	"fmt"
 	"github.com/Sirupsen/logrus"
-	"github.com/opentracing/opentracing-go"
 	"github.com/fnproject/fn/api/models"
 	"github.com/fnproject/fn/api/runner/common"
 	"github.com/fnproject/fn/api/runner/task"
+	"github.com/opentracing/opentracing-go"
 )
 
 func getTask(ctx context.Context, url string) (*models.Task, error) {
-	// TODO shove this ctx into the request?
+	ctx, log := common.LoggerWithFields(ctx, logrus.Fields{"runner": "async"})
 	span, _ := opentracing.StartSpanFromContext(ctx, "get_task")
 	defer span.Finish()
 
-	// TODO uh, make a better http client :facepalm:
-	resp, err := http.Get(url)
-	if err != nil {
-		return nil, err
-	}
+	req, _ := http.NewRequest("GET", url, nil)
+	resp, err := http.DefaultClient.Do(req.WithContext(ctx))
 	defer func() {
 		io.Copy(ioutil.Discard, resp.Body)
 		resp.Body.Close()
 	}()
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode != http.StatusOK {
+		return nil, errors.New(fmt.Sprintf("Unable to get task. Reason %v", resp.Status))
+	}
 
 	var task models.Task
 	err = json.NewDecoder(resp.Body).Decode(&task)
+	if err != nil {
+		log.WithError(err).Error("Unable to decode task from response object")
+	}
 	if err != nil {
 		return nil, err
 	}