Fix some quality issues, add .deepsource.toml

[allenjseb]

Background

DeepSource is a free for open-source tool that helps developers and teams write good code. It continuously analyzes code changes on every PR and gives a central dashboard to see code health in terms of issues and important metrics. DeepSource is used by teams at Uber, NASA and DGraph among others.
I've added a customized .deepsource.toml file for this repo, and made some fixes for issues existing in the code highlighted by DeepSource. To keep analyzing code on every change, integration is easy:

1. Merge this PR.
2. Sign up on DeepSource and grant access to this repository here.
3. Activate analysis for this repo here.
   You can also take a look at the docs. I'll be happy to answer any questions! :

Changes

1. Remove the unnecessary use of [:] in the following usage:

json := string(bytes[:])

2. Use an unused error.

Signed-off-by: Allen allen@deepsource.io