turbostat: Clean up error handling; disambiguate error messages; use err and errx

Most of turbostat's error handling consists of printing an error (often
including an errno) and exiting.  Since perror doesn't support a format
string, those error messages are often ambiguous, such as just showing a
file path, which doesn't uniquely identify which call failed.

turbostat already uses _GNU_SOURCE, so switch to the err and errx
functions from err.h, which take a format string.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Len Brown <len.brown@intel.com>