Ensures results are of type *multierror.Error and call ErrorOrNil at return

[JoshVanL]

What this PR does / why we need it:
Due to a result being miss returned a bug was created. This PR ensures all append results are of *multierror.Error and return ErrorOrNil()

NONE

[JoshVanL]

/assign @simonswine

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[simonswine]

Not too sure why we need that?! This is exactly as usage suggest https://github.com/hashicorp/go-multierror

[simonswine]

/assign @JoshVanL
/unassign

[jetstack-bot]

@JoshVanL: PR needs rebase.

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[jetstack-bot]

@JoshVanL: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
tarmak-puppet-module-tarmak-acceptance-1-14-centos	78a2add	link	/test puppet-tarmak-acceptance-centos v1.14

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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