refactor(package): replace `gulp-util` (deprecated) with `plugin-error` (`dependencies`)

[TheDancingCode]

Proposed Changes

gulp-util has been deprecated recently. Continuing to use this dependency may prevent the use of your library with the recently released version 4 of Gulp, it is important to replace gulp-util.

See:

• https://medium.com/gulpjs/gulp-util-ca3b1f9f9ac5
• https://github.com/gulpjs/gulp-util/issues/143

Types of Changes

What types of changes does your code introduce Put an x in the boxes that apply

• [x ] Bug (non-breaking change which fixes an issue)
• [ ] Feature (non-breaking change which adds functionality)
• [ ] Breaking Change (fix or feature which changes existing functionality)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is a reminder of what we are going to look for before merging your code.

• [x ] I have read the CONTRIBUTING guide
• [x ] Lint and unit tests pass with my changes
• [ ] I have added tests that prove my fix is effective/works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes are merged and published in downstream modules

Reviewers: @voischev, @michael-ciniawsky, @posthtml-collaborators

[coveralls]

Coverage remained the same at 87.097% when pulling 8f74e89b2bf68a7d84bd498444db4f17e967b65e on TheDancingCode:remove-gutil into 334e61590e962221825cc00cd32e8a69d7939666 on posthtml:master.

[coveralls]

Coverage remained the same at 87.097% when pulling 8f74e89b2bf68a7d84bd498444db4f17e967b65e on TheDancingCode:remove-gutil into 334e61590e962221825cc00cd32e8a69d7939666 on posthtml:master.

[coveralls]

Coverage remained the same at 87.097% when pulling 8f74e89b2bf68a7d84bd498444db4f17e967b65e on TheDancingCode:remove-gutil into 334e61590e962221825cc00cd32e8a69d7939666 on posthtml:master.