Test if error is nil before to log it

[nmengin]

Signed-off-by: nmengin nicolas@containo.us

- What I did

This PR adds a test on the error sent by the function store.CreateNetwork() when a manager becomes leader in the way to avoid the creation of an error message for a error which is nil.

- How I did it

The if statement test if the error is not nil before to log a message.

- How to test it

Start a swarmkit cluster with a log level >= logLevel.Error.
Thanks to the modification the errors described below sould not appear:

level=error msg="failed to create predefined network bridge" error="<nil>"
level=error msg="failed to create predefined network host" error="<nil>"

- Description for the changelog

Delete the error logs generated when prefefined networks are initiliazed successfully, Fixes #2719 .

[codecov]

Codecov Report

Merging #2720 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2720      +/-   ##
==========================================
- Coverage   61.99%   61.87%   -0.12%     
==========================================
  Files         134      134              
  Lines       21771    21771              
==========================================
- Hits        13496    13471      -25     
- Misses       6818     6850      +32     
+ Partials     1457     1450       -7

[thaJeztah]

LGTM

ping @fcrisciani

[fcrisciani]

LGTM

[dperny]

Wow, this is such a dumb oversight by us. Yeah, LGTM, much better, thanks.