Refactor getMachine to clarify machine-not-found error conditions

[clobrano]

Clarified the conditions under which an error would be returned or not
when the Machine was not found in the cluster.

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[clobrano]

Regarding this comment

I do think this is still a bit confusing because isUnhandledMachine value may not reflect the actual logic.
IIUC We only decide that the machine is unhandled after we also check ownerRef & remediation.
So to put it in other words isUnhandledMachine value may equal true when logically it is actually need to be false.

I think that we can consider a Machine "unhandled" even before checking ownerRef and remediation, as unhandled does not necessarily mean we can remediate it, so I went for renaming the mustExist variable in isUnhandledMachine

[clobrano]

/test 4.14-openshift-e2e

[slintes]

/lgtm

giving others the chance to review as well
/hold

[mshitrit]

/unhold

[clobrano]

/retest

[razo7]

/retest