update next.config.js target error message to be more clear

Author: lindsaylevine

index.js

@@ -57,7 +57,9 @@ module.exports = {
       const nextConfig = loadConfig(PHASE_PRODUCTION_BUILD, path.resolve('.'))
       const isValidTarget = acceptableTargets.includes(nextConfig.target)
       if (!isValidTarget) {
-        return failBuild(`next.config.js must be one of: ${acceptableTargets.join(', ')}`)
+        return failBuild(
+          `Your next.config.js must set the "target" property to one of: ${acceptableTargets.join(', ')}`,
+        )
       }
     } else {
       // Create the next config file with target set to serverless by default

test/index.js

@@ -161,7 +161,9 @@ describe('preBuild()', () => {
           utils,
           constants: { FUNCTIONS_SRC: 'out_functions' },
         }),
-      ).rejects.toThrow(`next.config.js must be one of: serverless, experimental-serverless-trace`)
+      ).rejects.toThrow(
+        `Your next.config.js must set the "target" property to one of: serverless, experimental-serverless-trace`,
+      )
     },
   )
 })