Add various missing error handling

... as found by (golangci-lint run).

Note: this does not add (golangci-lint run) to the Makefile
to ensure the coding standard.

(BTW golangci-lint currently fails on structcheck, which doesn't
handle embedded structs, and that's a years-long known unfixed
limitation.)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

integration/proxy_test.go

@@ -285,6 +285,9 @@ func runTestGetManifestAndConfig(p *proxy, img string) error {
 	}
 
 	_, err = p.callNoFd("CloseImage", []interface{}{imgid})
+	if err != nil {
+		return err
+	}
 
 	return nil
 }