Cherry-pick panic fix into v0.10.x

[ghost]

Changes

Fixes #2220

Cherry-picks #2222 into v0.10.x branch.

I'm making a PR just to see how integration runs against the backported patch.

Here's the original PR's blurb:

Panic in controller when step fails before image digest exporter

The image digest exporter (part of the Image Output Resource) is
configured with "terminationMessagePolicy": "FallbackToLogsOnError",.

When a previous step has failed in a Task our entrypoint wrapping the
exporter emits a log line like 2020/03/13 12:03:26 Skipping step because a previous step failed. Because the image digest exporter
is set to FallbackToLogsOnError Kubernetes slurps up this log line as
the termination message.

That line gets read by the Tekton controller, which is looking for JSON
in the termination message. It fails to parse and stops trying to read
step statuses.

That in turn results in a mismatch in the length of the list of steps and
the length of the list of step statuses. Finally we attempt to sort
the list of step statuses alongside the list of steps. This method
panics with an out of bounds error because it assumes the lengths of the two
lists are the same.

So, this PR does the following things:

1. The image digest exporter has the FallbackToLogsOnError policy
   removed. I can't think of a reason that we need this anymore.
2. The Tekton controller no longer breaks out of the loop while it's
   parsing step statuses and instead simply ignores non-JSON termination
   messages.

(cherry picked from commit 153f1d1)

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