nstat: print useful error messages in abort() cases

nstat: print useful error messages in abort() cases

When nstat temporary file is corrupted or in some other corner cases,
nstat use abort() to stop its execution. This can puzzle some users,
wondering what is the reason for the crash.

This commit replaces abort() with some meaningful error messages and exit()

Reported-by: Renaud Métrich <rmetrich@redhat.com>
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>