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/restorer/restorer_test.go

@@ -989,6 +989,9 @@ type printerMock struct {
 
 func (p *printerMock) Update(_ restoreui.State, _ time.Duration) {
 }
+func (p *printerMock) Error(item string, err error) error {
+	return nil
+}
 func (p *printerMock) CompleteItem(action restoreui.ItemAction, item string, size uint64) {
 }
 func (p *printerMock) Finish(s restoreui.State, _ time.Duration) {