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/terminal.go

@@ -0,0 +1,10 @@
+package ui
+
+// Terminal is used to write messages and display status lines which can be
+// updated. See termstatus.Terminal for a concrete implementation.
+type Terminal interface {
+	Print(line string)
+	Error(line string)
+	SetStatus(lines []string)
+	CanUpdateStatus() bool
+}