Revert "A nil context isn't an error"

This reverts commit fe9fe4c.

client/alloc_runner.go

@@ -51,14 +51,13 @@ type AllocRunner struct {
 
 	dirtyCh chan struct{}
 
-	ctx     *driver.ExecContext
-	ctxLock sync.Mutex
+	ctx        *driver.ExecContext
+	ctxLock    sync.Mutex
+	tasks      map[string]*TaskRunner
+	taskStates map[string]*structs.TaskState
+	restored   map[string]struct{}
+	taskLock   sync.RWMutex
 
-	tasks    map[string]*TaskRunner
-	restored map[string]struct{}
-	taskLock sync.RWMutex
-
-	taskStates     map[string]*structs.TaskState
 	taskStatusLock sync.RWMutex
 
 	updateCh chan *structs.Allocation
@@ -127,6 +126,9 @@ func (r *AllocRunner) RestoreState() error {
 	if r.alloc == nil {
 		snapshotErrors.Errors = append(snapshotErrors.Errors, fmt.Errorf("alloc_runner snapshot includes a nil allocation"))
 	}
+	if r.ctx == nil {
+		snapshotErrors.Errors = append(snapshotErrors.Errors, fmt.Errorf("alloc_runner snapshot includes a nil context"))
+	}
 	if e := snapshotErrors.ErrorOrNil(); e != nil {
 		return e
 	}