return errors properly in BitTube and SensorEventQueue

Change-Id: Idf31179f9fcc666e287c7b01b2007431cb60127d
diff --git a/libs/gui/BitTube.cpp b/libs/gui/BitTube.cpp
index 355a319..cf44bb9 100644
--- a/libs/gui/BitTube.cpp
+++ b/libs/gui/BitTube.cpp
@@ -140,8 +140,7 @@
         ssize_t size = tube->write(vaddr, objSize);
         if (size < 0) {
             // error occurred
-            numObjects = -size;
-            break;
+            return size;
         } else if (size == 0) {
             // no more space
             break;
@@ -160,8 +159,7 @@
         ssize_t size = tube->read(vaddr, objSize);
         if (size < 0) {
             // error occurred
-            numObjects = -size;
-            break;
+            return size;
         } else if (size == 0) {
             // no more messages
             break;