fix: handling when image contains no trivy-target

[Nikkely]

What did you implement:

I added error handling to trivy-to-vuls.
If Trivy scan the image that is not Trivy supported OS or Libraries, Vuls behave the result is not parsable and is error.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

1. write a test code
2. I have tried to parse result of ./trivy -q image --list-all-pkgs -f json hello-world:latest and confirmed outputting error as expected
3. I have tried to parse result of ./trivy -q image --list-all-packages -f json ubuntu:latest and confirmed outputing no error as usual

Checklist:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES

Reference

• https://blog.github.com/2015-01-21-how-to-write-the-perfect-pull-request/