Rename package srverrors to httperror

- Package names are preferred to be in singular form¹.
- Since errors are just for HTTP and not generic, I changed the base name to
'http'.
- I didn't name it simply 'error', because it would then conflict with the
- standard 'error' interface.

¹: https://rakyll.org/style-packages/

internal/server/httperror/error.go

@@ -0,0 +1,24 @@
+package httperror
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// A ServerErrMessage represents the reason why the error happened
+type ServerErrMessage struct {
+	Reason string `json:"reason"`
+}
+
+// ErrorJSON writes the provided error as a JSON body to the provided
+// http.ResponseWriter, after setting the appropriate Content-Type header
+func ErrorJSON(w http.ResponseWriter, error string, code int) {
+	body, _ := json.Marshal(
+		ServerErrMessage{
+			Reason: error,
+		},
+	)
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(code)
+	_, _ = w.Write(body)
+}

internal/server/httperror/error_test.go

@@ -0,0 +1,47 @@
+package httperror_test
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/BBVA/kapow/internal/server/httperror"
+)
+
+func TestErrorJSONSetsAppJsonContentType(t *testing.T) {
+	w := httptest.NewRecorder()
+
+	httperror.ErrorJSON(w, "Not Important Here", 0)
+
+	if v := w.Result().Header.Get("Content-Type"); v != "application/json; charset=utf-8" {
+		t.Errorf("Content-Type header mismatch. Expected: %q, got: %q", "application/json; charset=utf-8", v)
+	}
+}
+
+func TestErrorJSONSetsRequestedStatusCode(t *testing.T) {
+	w := httptest.NewRecorder()
+
+	httperror.ErrorJSON(w, "Not Important Here", http.StatusGone)
+
+	if v := w.Result().StatusCode; v != http.StatusGone {
+		t.Errorf("Status code mismatch. Expected: %d, got: %d", http.StatusGone, v)
+	}
+}
+
+func TestErrorJSONSetsBodyCorrectly(t *testing.T) {
+	expectedReason := "Something Not Found"
+	w := httptest.NewRecorder()
+
+	httperror.ErrorJSON(w, expectedReason, http.StatusNotFound)
+
+	errMsg := httperror.ServerErrMessage{}
+	if bodyBytes, err := ioutil.ReadAll(w.Result().Body); err != nil {
+		t.Errorf("Unexpected error reading response body: %v", err)
+	} else if err := json.Unmarshal(bodyBytes, &errMsg); err != nil {
+		t.Errorf("Response body contains invalid JSON entity: %v", err)
+	} else if errMsg.Reason != expectedReason {
+		t.Errorf("Unexpected reason in response. Expected: %q, got: %q", expectedReason, errMsg.Reason)
+	}
+}