error_displayable() currently calls variable_get(), which can't throw an exception - it has no dependencies at all.
If we convert that to CMI, then it could do all kinds of things, which isn't possible in at least two of these functions:
It could move to settings.php, but then there's no UI for setting it any more.
This is one of a small number of variables which don't comfortably sit as either raw settings.php, globals, config, state or cache since they were completely reliant on the dual-function of variable_get() working both before and after variable_init().
I don't have any ideas on how to fix this at the moment. Since it blocks eventual variable removal opening as 'major'.
FAILED: [[SimpleTest]]: [MySQL] 49,593 pass(es), 4 fail(s), and 0 exception(s).
PASSED: [[SimpleTest]]: [MySQL] 50,045 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 50,008 pass(es).