Add some missing error checks (#2420)

I found these with the 'errcheck' tool (via 'golangci-lint').

I aimed to apply reasonable judgement when deciding which errors
actually need handling, and how to handle them.

dnscrypt-proxy/config.go

@@ -861,7 +861,9 @@ func (config *Config) loadSources(proxy *Proxy) error {
 		}
 		proxy.registeredServers = append(proxy.registeredServers, RegisteredServer{name: serverName, stamp: stamp})
 	}
-	proxy.updateRegisteredServers()
+	if err := proxy.updateRegisteredServers(); err != nil {
+		return err
+	}
 	rs1 := proxy.registeredServers
 	rs2 := proxy.serversInfo.registeredServers
 	rand.Shuffle(len(rs1), func(i, j int) {

dnscrypt-proxy/main.go

@@ -27,8 +27,11 @@ type App struct {
 }
 
 func main() {
-	TimezoneSetup()
+	tzErr := TimezoneSetup()
 	dlog.Init("dnscrypt-proxy", dlog.SeverityNotice, "DAEMON")
+	if tzErr != nil {
+		dlog.Errorf("Timezone setup failed: %v", tzErr)
+	}
 	runtime.MemProfileRate = 0
 
 	seed := make([]byte, 8)
@@ -141,7 +144,9 @@ func (app *App) AppMain() {
 }
 
 func (app *App) Stop(service service.Service) error {
-	PidFileRemove()
+	if err := PidFileRemove(); err != nil {
+		dlog.Warnf("Failed to remove the PID file: %v", err)
+	}
 	dlog.Notice("Stopped.")
 	return nil
 }

dnscrypt-proxy/serversInfo.go

@@ -615,7 +615,9 @@ func dohTestPacket(msgID uint16) []byte {
 	msg.SetEdns0(uint16(MaxDNSPacketSize), false)
 	ext := new(dns.EDNS0_PADDING)
 	ext.Padding = make([]byte, 16)
-	crypto_rand.Read(ext.Padding)
+	if _, err := crypto_rand.Read(ext.Padding); err != nil {
+		dlog.Fatal(err)
+	}
 	edns0 := msg.IsEdns0()
 	edns0.Option = append(edns0.Option, ext)
 	body, err := msg.Pack()
@@ -638,7 +640,9 @@ func dohNXTestPacket(msgID uint16) []byte {
 	msg.SetEdns0(uint16(MaxDNSPacketSize), false)
 	ext := new(dns.EDNS0_PADDING)
 	ext.Padding = make([]byte, 16)
-	crypto_rand.Read(ext.Padding)
+	if _, err := crypto_rand.Read(ext.Padding); err != nil {
+		dlog.Fatal(err)
+	}
 	edns0 := msg.IsEdns0()
 	edns0.Option = append(edns0.Option, ext)
 	body, err := msg.Pack()