Whenever shelling out read both error and std streams for Notation client

[iamsamirzon]

This issue is created to replace a finding in #93. Refer example here . #83 (comment)

Desired Resolution

1. Search through all code in Notation, Notation-go, Notation-go-core and redirect stderr to console.

[yizha1]

Errors are already printed to stderr, so suggest closing this issue @iamsamirzon

[iamsamirzon]

@priteshbandi - Could you confirm and close this issue is fixed as expected?

[priteshbandi]

Issue is fixed https://github.com/notaryproject/notation-go/blob/8c3ed9217dc8025983f64a5510249a94dc009a86/plugin/plugin.go#L166-L177