Fix stderr obtention
Fix stderr obtention

* The Stderr pipe should be read before waiting
* The Stderr pipe should not be used with Run/Output. See https://golang.org/pkg/os/exec/#Cmd.StderrPipe:

  Wait will close the pipe after seeing the command exit, so most callers need
  not close the pipe themselves; however, an implication is that it is incorrect
  to call Wait before all reads from the pipe have completed. For the same
  reason, it is incorrect to use Run when using StderrPipe.

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