Provide a friendlier error message when YAML parsing fails

**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
> ```
> 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](https://kubernetes.slack.com/archives/CH8KCCKA5/p1632147054337500?thread_ts=1632129604.333300&cid=CH8KCCKA5))

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The issue does not name a file, test, or entry point. Locate ytt's YAML parsing error handling, then add wording that is understandable to non-experts while retaining the underlying parser error; add or update coverage for the reported invalid YAML case.

Written by the indexing model from the issue text.