tools/power/x86/intel-speed-select: Improve core-power result and error display

This change adds improved error display and handling for commands related
to core-power feature. The changes include:
- Replace perror with helpful error message
- Use ordered priority for SKX based platform by default as the
proportional priority is not supported
- Don't show weight and epp in help and also give error when user
tries to set them in SKX based platforms
- Range check for epp and weights and display error

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>