Add extra troubleshooting messages for errors and failures so that the
output can help troubleshoot the issue.

Make error handling more uniform by using -1 for errors (e.g. opening
files, reading from pagemap, etc.) and 1 for failures (data mismatch,
pages not COWed when expected to be, etc.)

Tested: Ran this test with criu versions known to have problems with COW
and with problems to read from pagemap due to the dumpable flag not
being preserved.
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>