Expect nice_err to fail on FreeBSD with unprivileged users

PR: 189821
Sponsored by: EMC / Isilon Storage Division
Check error return from reading integer part of temperature.

There's a very remote, but possible, chance that the integer part read will
fail, but the fraction read succeeds, at which point the reported temperature is
invalid.

Reported by: Matthew Rezny
MFC after:	3 weeks