Improving safe-init errors for promotion failure

[Xavientois]

We should improve the error message for promotion failure. (Here, this is warm because its outer is not initialized (Rule 1) but the QualifiedName class does access some fields (namely the cache at the top) of its outer (Rule 2). But the error message doesn't say anything about that.)

Originally posted by @olhotak in #14918 (comment)

[olhotak]

This was fixed by #15659. Closing.