Add covrage ignore on error paths

Author: barak007

lib/input.js

@@ -167,6 +167,7 @@ class Input {
       if (fileURLToPath) {
         result.file = fileURLToPath(fromUrl)
       } else {
+        // istanbul ignore next
         throw new Error(`file: protocol is not available in this PostCSS build`);
       }
     }

lib/map-generator.js

@@ -205,6 +205,7 @@ class MapGenerator {
       if (pathToFileURL) {
         return pathToFileURL(node.source.input.from).toString()
       } else {
+        // istanbul ignore next
         throw new Error('`map.absolute` option is not available in this PostCSS build')
       }
     } else {