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/mock.go

@@ -0,0 +1,22 @@
+package ui
+
+type MockTerminal struct {
+	Output []string
+	Errors []string
+}
+
+func (m *MockTerminal) Print(line string) {
+	m.Output = append(m.Output, line)
+}
+
+func (m *MockTerminal) Error(line string) {
+	m.Errors = append(m.Errors, line)
+}
+
+func (m *MockTerminal) SetStatus(lines []string) {
+	m.Output = append([]string{}, lines...)
+}
+
+func (m *MockTerminal) CanUpdateStatus() bool {
+	return true
+}