abrt-action-install-debuginfo: fix --exact handling. Fixes rhbz1027786

Before this change, running with
--exact=/usr/lib/debug/lib/modules/2.6.32-430.el6.i686/vmlinux
printed this:

Coredump references 0 debuginfo files, 1 of them are not installed
Setting up yum repositories
Looking for needed packages in repositories
Can't find packages for 1 debuginfo files
All debuginfo files are available

and exit code was 0. After this change:

1 of debuginfo files are not installed
Setting up yum repositories
Looking for needed packages in repositories
Can't find packages for 1 debuginfo files
Missing requested file: /usr/lib/debug/lib/modules/2.6.32-430.el6.i686/vmlinux

and exit code is 2.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>