listInterfaces() will fail to report errors from convertToInterfaces()

[dgibson]

Description of problem

Problem pointed out by @amshinde at #3003 (review)

In #3003 we allowed convertToInterfaces() to fail, but a bug in the implementation means that listInterfaces() doesn't properly propagate an error here to the caller.

Expected result

Errors reported by convertToInterfaces() are reported to the caller of listInterfaces().

Actual result

Errors reported by convertToInterfaces() will be lost and listInterfaces() will report an empty list and no error.

Further information

When looking at this, I noticed there's also a pre-existing bug where if the upcast to *grpc.Interfaces fails we'll also report no error. There are several other bugs with the same pattern in kata_agent.go. I'll try to address those too.