Tim noted while reviewing my work on MDL-42057 that the means by which we request renderers is inconsistent.
I felt it was a separate issue to mine and have opened this issue to look at it because it is a VERY valid point.

The following shows the different ways in which we request a core component renderer:

// To get a core component renderer:

$PAGE->get_renderer('core', 'course');

// Or....

$PAGE->get_renderer('core_course');

// To get a subtype of it it has to be.

$PAGE->get_renderer('core_course', 'management');

// To get the lesson renderer:

$PAGE->get_renderer('mod_lesson');

// To get a subtype of it it.

$PAGE->get_renderer('mod_lesson', 'management');

Really we should not allow the first way of getting a core renderer and for the time being show a debugging notice or similar.