v5.4: fix error message in private key existence check

[gerardsn]

aligned the message with v6.
In V6 Exists also returns an error clarifying this even further.

[stevenvegt]

I would suggest changing the interface to return the error instead. Helps us to debug in the future. Perhaps the vault service is down or some other thing has happened.

[gerardsn]

That would have to be a new issue that will eventually end up on the wontfix pile as this is already fixed in v6. I was just going for a message that is less deterministic about the cause of the failure.

[stevenvegt]

Since this is rewritten in v6, this should do the job for the remainder of the lifetime of the v5 branch.