Improve nullable annotations for Result.Error

[3Matvey]

This PR improves nullable flow analysis for the Result type.

Result already guarantees that Error is non-null when IsSuccess is false.
This change communicates that invariant to the compiler using
MemberNotNullWhen.

It also adds a null guard in the failure constructor to preserve the invariant
at runtime.

Before:

if (!result.IsSuccess)
    return result.Error!;

After:

if (!result.IsSuccess)
    return result.Error;