Bug 1403898

+++ This bug was initially created as a clone of Bug #1397427 +++

Description of problem:
When syncing channel from CDN using the cdn-sync command, there is possibility of missing RPM in the CDN. Cdn-sync should log explicit error in its log in such occasion, and exit with non-zero exit code. Currently it doesn't report any specific error in the log, and exits with 0 (success) exit code.

Version-Release number of selected component (if applicable):
spacewalk-backend-cdn-2.5.3-32

How reproducible:
Issue is present only when there is missing RPM file in the CDN (happens sometimes on the code-stage). As of today, there is missing libkadm5-1.14.1-26.el7-0.i686 in the rhel-x86_64-server-7 channel in the code-stage.

Steps to Reproduce:
1. cdn-sync -c rhel-x86_64-server-7

Actual results:

# cdn-sync -c rhel-x86_64-server-7
08:14:13 ======================================
08:14:13 | Channel: rhel-x86_64-server-7
08:14:13 ======================================
08:14:13 Sync of channel started.
08:14:13 Repo URL: https://cdn.stage.redhat.com/content/dist/rhel/server/7/7Server/x86_64/os
<...>
08:33:20 6307/13875 : libkadm5-1.14.1-26.el7-0.i686
08:33:21 failure: Packages/libkadm5-1.14.1-26.el7.i686.rpm from content_dist_rhel_server_7_7Server_x86_64_os: [Errno 256] No more mirrors to try.
08:33:21 ERROR: failure: Packages/libkadm5-1.14.1-26.el7.i686.rpm from content_dist_rhel_server_7_7Server_x86_64_os: [Errno 256] No more mirrors to try.
08:33:21 Repo URL: https://cdn.stage.redhat.com/content/dist/rhel/server/7/7.1/x86_64/kickstart
08:33:34 Packages in repo:              4371
<...>
08:35:59 Linking packages to channel.
08:36:23 ERROR: Could not find object [<spacewalk.server.importlib.importLib.IncompletePackage instance; attributes={'package_size': None, 'package_arch_id': 104, 'name': 'libkadm5', 'checksum_list': None, 'md5sum': None, 'org_id': None, 'epoch': '0', 'checksums': {'sha1': '553b168dc506d33f684f3a0c36f6054a3fa253b9'}, 'channels': {101: 'rhel-x86_64-server-7'}, 'nevra_id': 8689, 'package_id': None, 'last_modified': None, 'name_id': 3597, 'version': '1.14.1', 'checksum': '553b168dc506d33f684f3a0c36f6054a3fa253b9', 'release': '26.el7', 'checksum_type': 'sha1', 'arch': 'i686', 'evr_id': 5310, 'checksum_id': 773956}] in table rhnPackage
08:36:23 Sync of channel completed in 0:09:53.
08:36:23 Total time: 0:09:53

[root@hp-dl360g6-01 ~]# echo $?
0

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We have the same bug for spacewalk-repo-sync. It should return non-zero code if some problems occurred during syncing rpm-packages.