fmt: print parse errors on error

[lbajolet-hashicorp]

The fmt command reformats HCL2 templates, provided it can parse the file and reformat its contents according to the standards set by the HCL library's formatters.

However, if the file is malformed for some reason, the command will fail with a parse error, but while the parse error message is shown, the actual errors in the template(s) are not forwarded, making it hard for users to understand what went wrong with the contents of the file they're trying to format.

In order to be more helpful with those errors, we now forward those parsing errors to the UI.

[JenGoldstrich]

Tested it and read the code, LGTM!

Before:

packer fmt ~/malformed.pkr.hcl
Error: encountered an error while formatting /Users/()/malformed.pkr.hcl

failed to parse HCL /Users/()/malformed.pkr.hcl

After:

packer fmt ~/malformed.pkr.hcl
Error: encountered an error while formatting /Users/()/malformed.pkr.hcl

2 errors occurred:
        * failed to parse HCL /Users/()/malformed.pkr.hcl
        * /Users/()/malformed.pkr.hcl:5,1-8: Argument or block definition
required; An argument or block definition is required here. To set an argument,
use the equals sign "=" to introduce the argument value.

[nywilken]

Nice improvement. I left a small suggestion.

[nywilken]

Approving for when you are ready to merge.

Should this be backported to 1.10.x ?

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