chore: print to stderr

[MaineK00n]

What did you implement:

When an error occurs, use stderr instead of stdout.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

$ go-kev fetch kevuln --dbtype redis --dbpath "redis://127.0.0.1:6379" 1>/dev/null
EROR[05-16|11:45:32] failed to fetchKEVuln                    err="Failed to open DB. err: dial tcp 127.0.0.1:6379: connect: connection refused"
Failed to open DB. err: dial tcp 127.0.0.1:6379: connect: connection refused

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