Mark errf _Noreturn, and mark errf and warn __printflike

The _Noreturn attribute was added to placate Coverity and other static
analysis tools. The __printflike attribute was added to catch issues
with the calls related to printf(3) abuse.

• Modify the code to facilitate the __printflike attribute addition.
• Convert errf calls in to_mb(..) and to_mb_string(..) to warn(..) so the calls will return instead of exiting, as the code suggests it should.

Differential Revision: D10704
MFC after: 1 month
Reviewed by: pfg
Sponsored by: Dell EMC Isilon