ListConfig.__setitem__(slice, ...) does not roll-back cfg state upon mutation failure

[Jasha10]

Most mutations to OmegaConf containers are atomic in the sense that a failed mutation will result in an unmodified object. This is not the case, however, when ListConfig.__setitem__ is called on a slice.

To Reproduce
The last line of the script below demonstrates inconsistency (failure to roll-back ListConfig state on failed mutation).

from pytest import raises

from omegaconf import OmegaConf, UnsupportedValueType

illegal = object()

cfg = OmegaConf.create([1, 2, 3])

with raises(UnsupportedValueType):
    cfg.append(illegal)
assert cfg == [1, 2, 3]

with raises(UnsupportedValueType):
    cfg.extend([illegal])
assert cfg == [1, 2, 3]

with raises(UnsupportedValueType):
    cfg.insert(1, illegal)
assert cfg == [1, 2, 3]

with raises(UnsupportedValueType):
    cfg[1] = illegal
assert cfg == [1, 2, 3]

with raises(UnsupportedValueType):
    cfg[1:2] = [illegal]
assert cfg == [1, 3]  # Inconsistent

Expected behavior
The last line above should be assert cfg == [1, 2, 3]