Enable errchkjson checks

These are disabled by default, even when the linter itself is enabled.
This enables them and configures errcheck appropriately (json.Marshal
and json.MarshalIndent are checked by errchkjson and shouldn't be
checked by errcheck).

Signed-off-by: Stephen Kitt <[email protected]>

Author: skitt

.golangci.yml

@@ -3,6 +3,13 @@ version: "2"
 linters:
   default: none
   settings:
+    errcheck:
+      exclude-functions:
+        - encoding/json.Marshal
+        - encoding/json.MarshalIndent
+    errchkjson:
+      check-error-free-encoding: true
+      report-no-exported: true
     gocritic:
       disabled-checks:
         - ifElseChain