return an error when deleting a non-existent index or field

holder.go

@@ -371,10 +371,10 @@ func (h *Holder) DeleteIndex(name string) error {
 	h.mu.Lock()
 	defer h.mu.Unlock()
 
-	// Ignore if index doesn't exist.
+	// Confirm index exists.
 	index := h.index(name)
 	if index == nil {
-		return nil
+		return NotFoundError{ErrIndexNotFound}
 	}
 
 	// Close index.

index.go

@@ -346,10 +346,10 @@ func (i *Index) DeleteField(name string) error {
 	i.mu.Lock()
 	defer i.mu.Unlock()
 
-	// Ignore if field doesn't exist.
+	// Confirm field exists.
 	f := i.field(name)
 	if f == nil {
-		return nil
+		return NotFoundError{ErrFieldNotFound}
 	}
 
 	// Close field.

index_test.go

@@ -194,13 +194,14 @@ func TestIndex_DeleteField(t *testing.T) {
 		t.Fatal("expected nil field")
 	}
 
-	// Delete again to make sure it doesn't error.
-	if err := index.DeleteField("f"); err != nil {
+	// Delete again to make sure it errors.
+	err := index.DeleteField("f")
+	if err == nil || err.Error() != pilosa.ErrFieldNotFound.Error() {
 		t.Fatal(err)
 	}
 }
 
-// Ensure index can delete a field.
+// Ensure index can validate its name.
 func TestIndex_InvalidName(t *testing.T) {
 	path, err := ioutil.TempDir("", "pilosa-index-")
 	if err != nil {