Enable the "errchkjson" linter

[cbandy]

This is part of the bugs preset of golangci-lint. I want to enable this entire preset eventually.

The check-error-free-encoding setting is recommended upstream and simplifies code where json.Marshal is known to succeed.

Checklist:

• [x] Have you added an explanation of what your changes do and why you'd like them to be included?
• [x] Have you updated or added documentation for the change, as applicable?
• [x] Have you tested your changes on all related environments with successful results, as applicable?
  • [x] Have you added automated tests?

Type of Changes:

• [x] Testing enhancement
• [x] Other

[cbandy]

Some recent results compared to the ones in this PR.