use AddError everywhere (#2372)

plugins/inputs/powerdns/powerdns.go

@@ -41,7 +41,7 @@ func (p *Powerdns) Gather(acc telegraf.Accumulator) error {
 
 	for _, serverSocket := range p.UnixSockets {
 		if err := p.gatherServer(serverSocket, acc); err != nil {
-			return err
+			acc.AddError(err)
 		}
 	}
 

plugins/inputs/powerdns/powerdns_test.go

@@ -90,7 +90,7 @@ func TestMemcachedGeneratesMetrics(t *testing.T) {
 
 	var acc testutil.Accumulator
 
-	err = p.Gather(&acc)
+	err = acc.GatherError(p.Gather)
 	require.NoError(t, err)
 
 	intMetrics := []string{"corrupt-packets", "deferred-cache-inserts",