Fix _isFunctionDoesNotExist()

fixes motdotlaGH-840

Since the key of the error object changed from v3, it was not being checked correctly.

Author: abetomo

lib/main.js

@@ -950,7 +950,7 @@ they may not work as expected in the Lambda environment.
   }
 
   _isFunctionDoesNotExist (err) {
-    return err.code === 'ResourceNotFoundException' &&
+    return err.name === 'ResourceNotFoundException' &&
       !!err.message.match(/^Function not found:/)
   }
 

test/main.js

@@ -219,15 +219,15 @@ describe('lib/main', function () {
   describe('_isFunctionDoesNotExist', () => {
     it('=== true', () => {
       const err = {
-        code: 'ResourceNotFoundException',
+        name: 'ResourceNotFoundException',
         message: 'Function not found: arn:aws:lambda:XXX'
       }
       assert.isTrue(lambda._isFunctionDoesNotExist(err))
     })
 
     it('=== false', () => {
       const err = {
-        code: 'MissingRequiredParameter',
+        name: 'MissingRequiredParameter',
         message: 'Missing required key \'FunctionName\' in params'
       }
       assert.isFalse(lambda._isFunctionDoesNotExist(err))