Don't merge stderr to stdout

[g7r]

RunTerraformCommandAndGetStdoutE actually returns stdout and stderr combined. This commit fixes that.

Terragrunt and Terraform output their warnings to stderr. Commands like terraform output -json and terraform show -json are especially vulnerable to keeping their output clean.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.
• Make a plan for release of the functionality in this PR. If it delivers value to an end user, you are responsible for ensuring it is released promptly, and correctly. If you are not a maintainer, you are responsible for finding a maintainer to do this for you.

Release Notes (draft)

• fix: RunTerraformCommandAndGetStdoutE no longer merges stderr to its result.

[james03160927]

Failing test doesn't seem to be related to this change. LGTM

[james03160927]

LGTM

[james03160927]

Please "update branch" in order to merge this PR. @g7r

[james03160927]

LGTM