Certain cell failures are not detected

[ceball]

Inherited from nbval. E.g. test for bad magics is currently marked "known fail":

nbcelltests/nbcelltests/tests/test_test.py

Lines 475 to 481 in f6d2061

	# TODO: see https://github.com/computationalmodelling/nbval/issues/147
	class TestSomeSanity(_TestCellTests):
	NBNAME = BROKEN_MAGICS
	@pytest.mark.xfail(reason="error messages on shell channel are ignored by nbval")
	def test_bad_magic_does_let_everything_pass(self):

(note: typo in above method name - "does not")

There might be other cases too, haven't investigated.