[bug] pouch print error info twice when command was unknown

[zhuangqh]

Ⅰ. Issue Description

pouch print error info twice when command was unknown

Ⅱ. Describe what happened

➜  pouch git:(master) pouch foo
Error: unknown command "foo" for "pouch"

Did you mean this?
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Run 'pouch --help' for usage.
Error: unknown command "foo" for "pouch"

Did you mean this?
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Ⅲ. Describe what you expected to happen

➜  pouch git:(master) pouch foo
Error: unknown command "foo" for "pouch"

Did you mean this?
	top

Run 'pouch --help' for usage.

Ⅳ. How to reproduce it (as minimally and precisely as possible)

1. command pouch foo

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• pouch version (use pouch version):
  Version: 0.5.0
  APIVersion: 1.24
  Arch: amd64
  BuildTime: 2018-06-08T10:55:13+08:00
  GitCommit: 0.4.0-359-g74e5ac7-dirty
  GoVersion: go1.9.5
  KernelVersion: 4.13.0-43-generic
  Os: linux
• OS (e.g. from /etc/os-release):
  NAME="Ubuntu"
  VERSION="16.04.3 LTS (Xenial Xerus)"
  ID=ubuntu
  ID_LIKE=debian
  PRETTY_NAME="Ubuntu 16.04.3 LTS"
  VERSION_ID="16.04"
  HOME_URL="http://www.ubuntu.com/"
  SUPPORT_URL="http://help.ubuntu.com/"
  BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
  VERSION_CODENAME=xenial
  UBUNTU_CODENAME=xenial
• Kernel (e.g. uname -a):
  Linux vm 4.13.0-43-generic doc: update public email address for pouch team #48~16.04.1-Ubuntu SMP Thu May 17 12:56:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
• Install tools:
• Others: