change service#Start to return just error (Refs #45)

```
@melekes
yeah, bool is superfluous
@ethanfrey
If I remember correctly when I was writing test code, if I call Start() on a Service that is already running, it returns (false, nil). Only if I try to legitimately start it, but it fails in startup do I get an error.
The distinction is quite important to make it safe for reentrant calls. The other approach would be to have a special error type like ErrAlreadyStarted, then check for that in your code explicitly. Kind of like if I make a db call in gorm, and get an error, I check if it is a RecordNotFound error, or whether there was a real error with the db query.
@melekes
Ah, I see. Thanks. I must say I like ErrAlreadyStarted approach more (not just in Golang)
```

common/service.go

@@ -1,13 +1,19 @@
 package common
 
 import (
+	"errors"
 	"sync/atomic"
 
 	"github.com/tendermint/tmlibs/log"
 )
 
+var (
+	ErrAlreadyStarted = errors.New("already started")
+	ErrAlreadyStopped = errors.New("already stopped")
+)
+
 type Service interface {
-	Start() (bool, error)
+	Start() error
 	OnStart() error
 
 	Stop() bool
@@ -94,11 +100,11 @@ func (bs *BaseService) SetLogger(l log.Logger) {
 }
 
 // Implements Servce
-func (bs *BaseService) Start() (bool, error) {
+func (bs *BaseService) Start() error {
 	if atomic.CompareAndSwapUint32(&bs.started, 0, 1) {
 		if atomic.LoadUint32(&bs.stopped) == 1 {
 			bs.Logger.Error(Fmt("Not starting %v -- already stopped", bs.name), "impl", bs.impl)
-			return false, nil
+			return ErrAlreadyStopped
 		} else {
 			bs.Logger.Info(Fmt("Starting %v", bs.name), "impl", bs.impl)
 		}
@@ -106,12 +112,12 @@ func (bs *BaseService) Start() (bool, error) {
 		if err != nil {
 			// revert flag
 			atomic.StoreUint32(&bs.started, 0)
-			return false, err
+			return err
 		}
-		return true, err
+		return nil
 	} else {
 		bs.Logger.Debug(Fmt("Not starting %v -- already started", bs.name), "impl", bs.impl)
-		return false, nil
+		return ErrAlreadyStarted
 	}
 }
 

events/events_test.go

@@ -13,8 +13,8 @@ import (
 // listener to an event, and sends a string "data".
 func TestAddListenerForEventFireOnce(t *testing.T) {
 	evsw := NewEventSwitch()
-	started, err := evsw.Start()
-	if !started || err != nil {
+	err := evsw.Start()
+	if err != nil {
 		t.Errorf("Failed to start EventSwitch, error: %v", err)
 	}
 	messages := make(chan EventData)
@@ -33,8 +33,8 @@ func TestAddListenerForEventFireOnce(t *testing.T) {
 // listener to an event, and sends a thousand integers.
 func TestAddListenerForEventFireMany(t *testing.T) {
 	evsw := NewEventSwitch()
-	started, err := evsw.Start()
-	if !started || err != nil {
+	err := evsw.Start()
+	if err != nil {
 		t.Errorf("Failed to start EventSwitch, error: %v", err)
 	}
 	doneSum := make(chan uint64)
@@ -62,8 +62,8 @@ func TestAddListenerForEventFireMany(t *testing.T) {
 // of the three events.
 func TestAddListenerForDifferentEvents(t *testing.T) {
 	evsw := NewEventSwitch()
-	started, err := evsw.Start()
-	if !started || err != nil {
+	err := evsw.Start()
+	if err != nil {
 		t.Errorf("Failed to start EventSwitch, error: %v", err)
 	}
 	doneSum := make(chan uint64)
@@ -107,8 +107,8 @@ func TestAddListenerForDifferentEvents(t *testing.T) {
 // for each of the three events.
 func TestAddDifferentListenerForDifferentEvents(t *testing.T) {
 	evsw := NewEventSwitch()
-	started, err := evsw.Start()
-	if !started || err != nil {
+	err := evsw.Start()
+	if err != nil {
 		t.Errorf("Failed to start EventSwitch, error: %v", err)
 	}
 	doneSum1 := make(chan uint64)
@@ -167,8 +167,8 @@ func TestAddDifferentListenerForDifferentEvents(t *testing.T) {
 // the listener and fires a thousand integers for the second event.
 func TestAddAndRemoveListener(t *testing.T) {
 	evsw := NewEventSwitch()
-	started, err := evsw.Start()
-	if !started || err != nil {
+	err := evsw.Start()
+	if err != nil {
 		t.Errorf("Failed to start EventSwitch, error: %v", err)
 	}
 	doneSum1 := make(chan uint64)
@@ -212,8 +212,8 @@ func TestAddAndRemoveListener(t *testing.T) {
 // TestRemoveListener does basic tests on adding and removing
 func TestRemoveListener(t *testing.T) {
 	evsw := NewEventSwitch()
-	started, err := evsw.Start()
-	if !started || err != nil {
+	err := evsw.Start()
+	if err != nil {
 		t.Errorf("Failed to start EventSwitch, error: %v", err)
 	}
 	count := 10
@@ -265,8 +265,8 @@ func TestRemoveListener(t *testing.T) {
 // `go test -race`, to examine for possible race conditions.
 func TestRemoveListenersAsync(t *testing.T) {
 	evsw := NewEventSwitch()
-	started, err := evsw.Start()
-	if !started || err != nil {
+	err := evsw.Start()
+	if err != nil {
 		t.Errorf("Failed to start EventSwitch, error: %v", err)
 	}
 	doneSum1 := make(chan uint64)