Merge branch 'davewasmer-patch-1'

Author: adamwathan

src/lib/substituteVariantsAtRules.js

@@ -78,6 +78,11 @@ export default function(config, { variantGenerators: pluginVariantGenerators })
       }
 
       _.forEach(_.without(ensureIncludesDefault(variants), 'responsive'), variant => {
+        if (!variantGenerators[variant]) {
+          throw new Error(
+            `Your config mentions the "${variant}" variant, but "${variant}" doesn't appear to be a variant. Did you forget or misconfigure a plugin that supplies that variant?`
+          )
+        }
         variantGenerators[variant](atRule, config)
       })