fix indentation if exception is raised inside `indent`

for example suppose such code
```ruby
h = HighLine.new
h.say('some line')
begin
    h.indent {
        h.say(1 / 0)
    }
rescue
end
h.say('another line')
```

this PR fixes it so that `another line` won't be indented. Currently it would be indented because exception would happen before `indent` could revert state.