Assignment of invalid value to ListConfig modifies the value.

[pereman2]

Describe the bug
Assigning a list with invalid values to a ListConfig keeps the content modified after raising the error.
To Reproduce

    @dataclass
    class ListClass:
        list: List[int] = field(default_factory=lambda: [1])
    cfg = OmegaConf.structured(ListClass)
    try:
        cfg.list = [1, 2, "foo"]
    except:
        pass
    print(cfg.list) # output [1, 2]

Expected behavior
cfg.list should return to be [1]
Additional context

• OmegaConf version: 2.1.0
• Python version: 3.8
• Operating system : macOS
• Please provide a minimal repro