fix: report error when creating a new network with an existent name

[ZouRui89]

Signed-off-by: Zou Rui [email protected]

1.Describe what this PR did
Report error message when creating a new network with an existent name.
2.Does this pull request fix one issue?

fixes #662
3.Describe how you did it
Before creating a new network, check if the network name exists.
4.Describe how to verify it
$ pouch network create net1
net1: b8d22aaa9db862f9f393eca9f4bdb45b12539b6352d417456bf4fd7774f3e2de
$ pouch network create net1
Error: {"message":"network net1 already exists"}

5.Special notes for reviews

[allencloud]

Please add an integration test case. Thanks a lot. @ZouRui89
And check if it returns a status code of 409.

[codecov-io]

Codecov Report

Merging #663 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #663   +/-   ##
=======================================
  Coverage   10.16%   10.16%           
=======================================
  Files          71       71           
  Lines        3425     3425           
=======================================
  Hits          348      348           
  Misses       3038     3038           
  Partials       39       39

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[ZouRui89]

It still returns 500. Please take a look.@allencloud

[allencloud]

LGTM, thanks a lot for your work. @ZouRui89