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.

changelog/unreleased/issue-4944

@@ -0,0 +1,8 @@
+Enhancement: Print JSON-formatted errors during `restore --json`
+
+Restic printed any restore errors directly to the console as freeform
+text messages, even with `--json`. Restic now prints them as JSON formatted
+messages when `--json` is passed.
+
+https://github.com/restic/restic/issues/4944
+https://github.com/restic/restic/pull/4946

changelog/unreleased/issue-4945

@@ -0,0 +1,8 @@
+Bugfix: Include missing backup error text with `--json`
+
+Restic was not actually providing the text of an error message during
+backup if `--json` was passed, instead only printing `"error": {}`.
+Restic now includes the error text in JSON output.
+
+https://github.com/restic/restic/issues/4945
+https://github.com/restic/restic/pull/4946