Try to warm all images and warn about errors

[m4x1202]

Description

When providing multiple images to warm, the warmer stops at the first error.
There are use cases however where an image is provided to the warmer which does not yet exist on purpose (e.g. CI/CD latest image only created at first release).
Thus we want to try all given images and only error if none of them work.

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Release Notes

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- kaniko tries to warm all given images and only errors if none are working

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[m4x1202]

@googlebot I signed it!

[m4x1202]

@tejal29 thanks for the approval :)
Can I do something else to help this PR?
Unfortunately cannot find a button/command to retrigger the Travis build. Based on what you said in a different PR the pipeline errors should be fixed.