refactor(sequence+selector): pass through errors unmodified

Any error values will not be munged directly by the Sequence or Selector
implementations of Tick. The primary reason for this removal was it's general
uselessness vs the amount of potential log spam that may be generated, when
propagating an error upwards through a large tree.

I also noticed I had flipped the comments somehow, oops.

any_test.go

@@ -137,7 +137,7 @@ func ExampleAny_resetBehavior() {
 	//1
 	//failure <nil>
 	//1
-	//failure behaviortree.Sequence encountered error with child at index 0: some_error
+	//failure some_error
 	//1
 	//2
 	//success <nil>
@@ -282,7 +282,7 @@ func TestAny_nilChildTick(t *testing.T) {
 	if status != Failure {
 		t.Error(status)
 	}
-	if err == nil || err.Error() != `behaviortree.Sequence encountered error with child at index 0: behaviortree.Node cannot tick a node with a nil tick` {
+	if err == nil || err.Error() != `behaviortree.Node cannot tick a node with a nil tick` {
 		t.Error(err)
 	}
 }

selector.go

@@ -16,15 +16,13 @@
 
 package behaviortree
 
-import "fmt"
-
-// Selector is a tick implementation that will succeed if no children fail, returning running if any children return
-// running, propagating any error
+// Selector is a tick implementation that ticks each child sequentially, until the the first error (returning the
+// error), the first non-failure status (returning the status), or all children are ticked (returning failure)
 func Selector(children []Node) (Status, error) {
-	for i, c := range children {
+	for _, c := range children {
 		status, err := c.Tick()
 		if err != nil {
-			return Failure, fmt.Errorf("behaviortree.Selector encountered error with child at index %d: %s", i, err.Error())
+			return Failure, err
 		}
 		if status == Running {
 			return Running, nil

sequence.go

@@ -16,15 +16,13 @@
 
 package behaviortree
 
-import "fmt"
-
-// Sequence is a tick implementation that will succeed if any children succeed, returning running if any children
-// return running, propagating any error
+// Sequence is a tick implementation that ticks each child sequentially, until the the first error (returning the
+// error), the first non-success status (returning the status), or all children are ticked (returning success)
 func Sequence(children []Node) (Status, error) {
-	for i, c := range children {
+	for _, c := range children {
 		status, err := c.Tick()
 		if err != nil {
-			return Failure, fmt.Errorf("behaviortree.Sequence encountered error with child at index %d: %s", i, err.Error())
+			return Failure, err
 		}
 		if status == Running {
 			return Running, nil