fix: ensure plugin error is emitted only once

lib/index.js

@@ -12,14 +12,17 @@ const genCustomBlocksCode = require('./codegen/customBlocks')
 const componentNormalizerPath = require.resolve('./runtime/componentNormalizer')
 const { NS } = require('./plugin')
 
+let errorEmitted = false
+
 module.exports = function (source) {
   const loaderContext = this
 
-  if (!loaderContext['thread-loader'] && !loaderContext[NS]) {
+  if (!errorEmitted && !loaderContext['thread-loader'] && !loaderContext[NS]) {
     loaderContext.emitError(new Error(
       `vue-loader was used without the corresponding plugin. ` +
       `Make sure to include VueLoaderPlugin in your webpack config.`
     ))
+    errorEmitted = true
   }
 
   const stringifyRequest = r => loaderUtils.stringifyRequest(loaderContext, r)