ShowDocSpec: fix indentation

"The snake which cannot cast its skin has to die.
As well the minds which are prevented from changing
their opinions; they cease to be mind."

  - Friedrich Nietzsche
ShowDoc: don't display header on empty docs

Fix issue #836 (`show-doc` "displays" nonexistent docs)

Instead of displaying a header without a comment, raise `CommandError`
and notify the user that a comment for the given definition doesn't
exist (but the very definition is present).

Add a test for this change. Also amend a couple of older tests, so they
conform the new behaviour.