[tz] tzcode error handling

[Paul Eggert]

On 12/11/2017 04:16 PM, Robert Elz wrote:
> simply add some error/status info into the internal zone
> structure, and then add a function to extract status (much like ferror()
> does for stdio - though perhaps with a little more info available).

Perhaps this could be done by having localtime etc. set tm_isdst to a 
negative value when there's a configuration error. If we're feeling 
lazy, we could use strerror (- tm->tm_isdst) to stringify the error 
indication.