Logging `BedrockError.cause` needs to be improved

[mattcollier]

https://github.com/digitalbazaar/bedrock/blob/eaac0b907fcbb8c47ed70977efbeb7a781345c0a/lib/util.js#L80-L82

This code assumes that every cause would be a BedrockError that would be explicitly marked as public.

It is not common practice to wrap every error that might be included in a BedrockError in another BedrockError.

I think the practical solution here is to say that the cause should be logged if the outer BedrockError is marked as public.

This of course could mean that a cause leaks data that should not be exposed in logs. However, the current state of affairs makes it impossible to debug applications issues in deployments.