Forwarded attributes can crash generator when accessing invalid members #683

Describe the bug

The forwarded attribute logic (for both observable properties and relay commands) can crash the generator in some invalid scenarios. We should instead emit a proper diagnostic and gracefully skip the current attribute, rather than crashing the generator.

Steps to reproduce

public partial class Foo : ObservableObject
{
    [ObservableProperty]
    [property: Test(TestAttribute.M)]
    private int number;
}

public class TestAttribute : Attribute
{
    public static string M => "";
}

Expected behavior

This should skip the attribute and generate a diagnostic.