daemon: replace error with warning message when synchronize snapshot update

[chuanchang]

The Sync() is an loop function, which will collect usage of snapshot util
success, it means there are many error message will be written into log
if it's failed to get usage for snapshot, these error will be easy to
trigger a monitor tool, so replace writing error with warning into log.

Signed-off-by: Alex Jia chuanchang.jia@gmail.com

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

e.g.

time="2018-12-28T06:14:25.311286459+08:00" level=error msg="failed to get usage for snapshot "0781c8f0ccb03a1341fb715a7ba6e615308bcf584ccd37e47efef4a73da03034":lstat /home/t4/pouch/containerd/root/io.containerd.snapshotter.v1.overlayfs/snapshots/6904/fs/usr/local/ilogtail/snapshot/ilogtail_status.LOG.44: no such file or directory: not found"
time="2018-12-28T07:07:31.03424663+08:00" level=error msg="failed to get usage for snapshot "fb280451f04f57d476838b718c520f68f1d834669eb91cb9b8f6c53fa6384455": lstat /home/t4/pouch/containerd/root/io.containerd.snapshotter.v1.overlayfs/snapshots/8181/fs/usr/local/ilogtail/snapshot/ilogtail_status.LOG.12: no such file ordirectory: not found"
time="2018-12-28T08:12:55.321965671+08:00" level=error msg="failed to get usage for snapshot "2340109eab02355a7738a761284af455b00db33813fbe635e1b1e6b212a3025a":lstat /home/t4/pouch/containerd/root/io.containerd.snapshotter.v1.overlayfs/snapshots/7962/fs/usr/local/ilogtail/snapshot/ilogtail_status.LOG.39: no such file or directory: not found"
time="2018-12-28T08:59:20.510390419+08:00" level=error msg="failed to get usage for snapshot "7de2a771e56621b6904d38cd99359bf9972e25c5e824e44921e1c03932cc1faf":lstat /home/t4/pouch/containerd/root/io.containerd.snapshotter.v1.overlayfs/snapshots/7938/fs/usr/local/ilogtail/snapshot/ilogtail_status.LOG.43: no such file or directory: not found"
time="2018-12-28T09:29:30.133803579+08:00" level=error msg="failed to get usage for snapshot "947431cc154292676ebaeb003070d0ea9656a25ec846bd5deb5fb81642d03910":lstat /home/t4/pouch/containerd/root/io.containerd.snapshotter.v1.overlayfs/snapshots/4922/fs/tmp/logtail_check_point.bak: no such file or directory: not found"
time="2018-12-28T10:10:55.484146651+08:00" level=error msg="failed to get usage for snapshot "2340109eab02355a7738a761284af455b00db33813fbe635e1b1e6b212a3025a":lstat /home/t4/pouch/containerd/root/io.containerd.snapshotter.v1.overlayfs/snapshots/7962/fs/usr/local/ilogtail/snapshot/ilogtail_status.LOG.41: no such file or directory: not found"

[codecov]

Codecov Report

Merging #2629 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #2629      +/-   ##
=========================================
- Coverage    69.5%   69.5%   -0.01%     
=========================================
  Files         280     280              
  Lines       18844   18844              
=========================================
- Hits        13098   13097       -1     
+ Misses       4279    4278       -1     
- Partials     1467    1469       +2

Flag	Coverage Δ
#criv1alpha1test	31.36% <0%> (ø)	⬆️
#criv1alpha2test	35.66% <0%> (ø)	⬆️
#integrationtest	41.68% <0%> (+0.01%)	⬆️
#nodee2etest	32.68% <0%> (-0.03%)	⬇️
#unittest	26.91% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
daemon/mgr/snapshot.go	33.33% <0%> (ø)	⬆️
cri/ocicni/cni_manager.go	58.82% <0%> (-11.77%)	⬇️
ctrd/container.go	58.1% <0%> (-0.8%)	⬇️
cri/v1alpha2/cri.go	67.69% <0%> (-0.13%)	⬇️
cri/v1alpha2/cri_wrapper.go	66.8% <0%> (+1.2%)	⬆️
ctrd/image.go	78.18% <0%> (+2.27%)	⬆️
apis/server/utils.go	75% <0%> (+3.84%)	⬆️

[allencloud]

LGTM