restore: print JSON versions of errors in --json mode

Previously, they were printed as freeform text.

This also adds a ui.Terminal interface to make writing
tests easier and also adds a few tests.

internal/ui/backup/text_test.go

@@ -0,0 +1,27 @@
+package backup
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/test"
+	"github.com/restic/restic/internal/ui"
+)
+
+func createTextProgress() (*ui.MockTerminal, ProgressPrinter) {
+	term := &ui.MockTerminal{}
+	printer := NewTextProgress(term, 3)
+	return term, printer
+}
+
+func TestError(t *testing.T) {
+	term, printer := createTextProgress()
+	test.Equals(t, printer.Error("/path", errors.New("error \"message\"")), nil)
+	test.Equals(t, []string{"error: error \"message\"\n"}, term.Errors)
+}
+
+func TestScannerError(t *testing.T) {
+	term, printer := createTextProgress()
+	test.Equals(t, printer.ScannerError("/path", errors.New("error \"message\"")), nil)
+	test.Equals(t, []string{"scan: error \"message\"\n"}, term.Errors)
+}