fix(index): emit warnings as an instance of {Error} instead of {String}

[maxleaver]

Warnings are currently sent to webpack as strings. However, Webpack's emitWarning function expects an error, as described in their API. This results in the errors described in #350. This PR simply changes the warnings emitted by postcss-loader from strings to an error instance.

Type

• Fix

SemVer

• Bug (:label: Patch)

Issues

• Fixes postcss warnings print "(Emitted value instead of an instance of Error)" #350

Checklist

• 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

[alexander-akait]

Thanks for PR! Can you add test?

[alexander-akait]

/cc @Kovensky can you test this too, thanks

[michael-ciniawsky]

Please add a test for this. Feel free to ask any question in the PR thread in case you're stuck with the test setup. test/Errors.test.js(Syntax Error) is a good starting point

[michael-ciniawsky]

I need to check the Postcss API docs, but I think warnings can contain location info and a code frame if the plugin author used the appropiated PostCSS Plugin API. If that's the case it would be better to reuse the code located in lib/Error.js and create a new class LoaderWarning (lib/Warning.js) based upon that to be consistent in terms of formatting

[michael-ciniawsky]

Closing in favor of 6eb82be Thx