linter: (1/2) enable errcheck (#5064)

## Description

partially cleanup in preparation for errcheck

i ignored a bunch of defer errors in tests but with the update to go 1.14 we can use `t.Cleanup(func() { if err := <>; err != nil {..}}` to cover those errors, I will do this in pr number two of enabling errcheck.

ref #5059

tools/tm-signer-harness/internal/test_harness_test.go

@@ -161,7 +161,7 @@ func harnessTest(t *testing.T, signerServerMaker func(th *TestHarness) *privval.
 	ss := signerServerMaker(th)
 	require.NoError(t, ss.Start())
 	assert.True(t, ss.IsRunning())
-	defer ss.Stop()
+	defer ss.Stop() //nolint:errcheck // ignore for tests
 
 	<-donec
 	assert.Equal(t, expectedExitCode, th.exitCode)