incorrect error message when asset gear is misconfigured

[hidden-dimensions]

(Describe the issue here)

• Skygear Server Date/Version: 0135aa2

When configured S3 with incorrect region and attempt to upload, the error message is misleading.

Expected Results

Should display a meaning error message.

Actual Results

{"error":{"name":"AlreadyExists","reason":"DuplicateAsset","message":"duplicate asset","code":409}}

Steps to reproduce

1. config asset gear with incorrect S3 region (probably other settings as well)

curl -X POST -d '{
  "access": "private",
  "headers": {
     "content-type": "image/jpeg",
     "content-length": "10"
  }
}' http://localhost:3000/_asset/presign_upload \
  -H "x-skygear-api-key: *** \
  -H "content-type: application/json"