save start error into State.Error

when a container failed to start, saves the error message into State.Error so
that it can be retrieved when calling `docker inspect` instead of having to
look at the log

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)

daemon/container.go

@@ -297,6 +297,8 @@ func (container *Container) Start() (err error) {
 	// setup has been cleaned up properly
 	defer func() {
 		if err != nil {
+			container.setError(err)
+			container.toDisk()
 			container.cleanup()
 		}
 	}()

daemon/state.go

@@ -15,6 +15,7 @@ type State struct {
 	Restarting bool
 	Pid        int
 	ExitCode   int
+	Error      string // contains last known error when starting the container
 	StartedAt  time.Time
 	FinishedAt time.Time
 	waitChan   chan struct{}
@@ -137,6 +138,7 @@ func (s *State) SetRunning(pid int) {
 }
 
 func (s *State) setRunning(pid int) {
+	s.Error = ""
 	s.Running = true
 	s.Paused = false
 	s.Restarting = false
@@ -179,6 +181,13 @@ func (s *State) SetRestarting(exitCode int) {
 	s.Unlock()
 }
 
+// setError sets the container's error state. This is useful when we want to
+// know the error that occurred when container transits to another state
+// when inspecting it
+func (s *State) setError(err error) {
+	s.Error = err.Error()
+}
+
 func (s *State) IsRestarting() bool {
 	s.Lock()
 	res := s.Restarting