Show code snippets on demand (#7440)
* Start working on source code snippets
* Make some cleanup
* Add couple tests
* Undo the dedicated error; update self-check to see these columns
* Only do wrapping when showing source
* Minor fixes
* Fix tests on Python 3.5
* Support also blocking errors and daemon
* Add couple tests
* Fix bug
* Address some CR
* More CR
* Separate fit in terminal from colorizing
* Don't mutate message lists in-place
* Update tests
* Fix couple of-by-one errors; add tests and comments
* Small tweaks
* Add an end-to-end daemon test for --pretty
