NuGetSigner ignores SignAsync return value

[zooba]

When signing a nupkg fails, RetryingSigner.SignAsync returns false.

However, the NuGetSigner (subclass) which calls this function ignores the result.

As a result, the sign tool exits with exit code 0 and the caller does not know that signing has failed.

[zooba]

It looks like this also affects VsixSigner.

However, ClickOnceSigner converts the result into a generic exception.