Rename KUBERNETES_EXTERNAL_DIFF to KUBECTL_EXTERNAL_DIFF

[apelisse]

And print an error when the diff invocation fails. It is a problem today, because if you specify an alternate binary that can't even be found, kubectl will leave with a success, even though nothing happened.
One of the drawback of doing that is that any non-empty diff will also print "exit status 1", which I think is mostly fine. It also helps whatever script is using kubectl diff to see if something needs to be applied or not.

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Release note:

NONE

[apelisse]

/assign @soltysh @seans3

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: apelisse

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• pkg/kubectl/OWNERS [apelisse]
• test/cmd/OWNERS [apelisse]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[apelisse]

/kind bug

[seans3]

/lgtm

[fejta-bot]

/retest
This bot automatically retries jobs that failed/flaked on approved PRs (send feedback to fejta).

Review the full test history for this PR.

Silence the bot with an /lgtm cancel comment for consistent failures.

[fejta-bot]

/retest
This bot automatically retries jobs that failed/flaked on approved PRs (send feedback to fejta).

Review the full test history for this PR.

Silence the bot with an /lgtm cancel comment for consistent failures.

[fejta-bot]

/retest
This bot automatically retries jobs that failed/flaked on approved PRs (send feedback to fejta).

Review the full test history for this PR.

Silence the bot with an /lgtm cancel comment for consistent failures.