libgui: add some error checks

This change adds a few error checks both in the framework and in some tests.

Change-Id: I2baf2676942a0dc15866e75852a775a0091ed16d
diff --git a/libs/gui/SurfaceTexture.cpp b/libs/gui/SurfaceTexture.cpp
index 9ed23be..975eb23 100644
--- a/libs/gui/SurfaceTexture.cpp
+++ b/libs/gui/SurfaceTexture.cpp
@@ -288,7 +288,9 @@
         computeCurrentTransformMatrix();
     } else  {
         if (err < 0) {
-            ST_LOGE("updateTexImage failed on acquire %d", err);
+            ST_LOGE("updateTexImage: acquire failed: %s (%d)",
+                strerror(-err), err);
+            return err;
         }
         // We always bind the texture even if we don't update its contents.
         glBindTexture(mTexTarget, mTexName);