lint: treat logEvent.AddError as a formatter (#6526)

Also fix a couple of places that called fmt.Sprintf and err.Error()
redundantly.

.golangci.yml

@@ -31,6 +31,7 @@ linters-settings:
           - (github.com/letsencrypt/boulder/log.Logger).AuditInfof
           - (github.com/letsencrypt/boulder/log.Logger).AuditErrf
           - (github.com/letsencrypt/boulder/ocsp/responder).SampledError
+          - (github.com/letsencrypt/boulder/web.RequestEvent).AddError
   staticcheck:
     # SA1019: Using a deprecated function, variable, constant or field
     # SA6003: Converting a string to a slice of runes before ranging over it

wfe2/wfe.go

@@ -328,7 +328,7 @@ func (wfe *WebFrontEndImpl) writeJsonResponse(response http.ResponseWriter, logE
 		// Don't worry about returning this error because the caller will
 		// never handle it.
 		wfe.log.Warningf("Could not write response: %s", err)
-		logEvent.AddError(fmt.Sprintf("failed to write response: %s", err))
+		logEvent.AddError("failed to write response: %s", err)
 	}
 	return nil
 }