add more descriptive error

Author: swyxio

Diff for: lib/build.js

@@ -54,9 +54,10 @@ function webpackConfig(dir, {userWebpackConfig, useBabelrc} = {}) {
   if (dirPath === functionsPath) {
     throw new Error(
       `
-      netlify-lambda Error: Function source folder (specified in netlify-lambda serve/build command) 
-      and publish folder (specified in netlify.toml)
-      should be different. 
+      netlify-lambda Error: Function source folder (specified in netlify-lambda serve/build command) and publish folder (specified in netlify.toml)
+      should be different. They are both set to ${dirPath}.
+
+      This is a common mistake for people switching from Netlify Dev to netlify-lambda. For an easy fix, change your functions key inside netlify.toml to something else, like "functions-build".
       For more info, check https://github.com/netlify/netlify-lambda#usage
       `
     );