fix(scan): ignore wp-cli stderr messages (#825)

[shigechika]

If this Pull Request is work in progress, Add a prefix of “[WIP]” in the title.

What did you implement:

vuls scan failed that wordpress site includes poor theme/plugin.
when wp-cli output stderr, vuls scan can't get only json messages.

Fixes #825

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?

I checked to ignore the following messages.

Unable to open /SITE1/wordpress/wp-content/wflogs/ips.php for reading and writing.
PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, function 'goyahso_change_post_object' not found or invalid function name in /SITE1/wordpress/wp-includes/class-wp-hook.php on line 286
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'goyahso_change_post_object' not found or invalid function name in /SITE1/wordpress/wp-includes/class-wp-hook.php on line 286

PHP Warning:  "continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"? in /SITE2/wordpress/wp-includes/pomo/plural-forms.php on line 210
Warning: "continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"? in /SITE2/wordpress/wp-includes/pomo/plural-forms.php on line 210

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/

[kotakanbe]

Thanks!