Fixed: set errored to false if warning no contains errors.

#22

BTW, i would recommend don't changed `stylelintResult`, just add node in `README.md` about disable `no-empty-source` and `no-missing-end-of-source-newline`, in next version we can change `stylelintResult` (example rename `warnings` to another name) and `stylelint-processor-styled-components` will cease to work as expected.

Author: evilebottnawi

src/index.js

@@ -95,6 +95,10 @@ module.exports = (/* options */) => ({
       prevWarnings.push(correctedWarning)
       return prevWarnings
     }, [])
+    
+    if (newWarnings.length === 0) {
+      stylelintResult.errored = false
+    }
 
     return Object.assign(stylelintResult, { warnings: newWarnings })
   },