Provide a friendlier error message when YAML parsing fails

[pivotaljohn]

Describe the problem/challenge you have

I did a mistake as I copied the lines from the *.star file which is indeed not a yaml file.
Nevertheless, the wording reported:

ytt: Error: Unmarshaling YAML template 'values.yml': yaml: line 12: did not find expected <document start>

is not really helpfull

https://kubernetes.slack.com/archives/CH8KCCKA5/p1632129604333300

Describe the solution you'd like
Provide an error message that makes it understandable to non yaml experts but also gives the actual underlying error which could be helpful for more advanced "yaml engineers". (ref)

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