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/ui/GraphicBuffer.cpp b/libs/ui/GraphicBuffer.cpp
index 57063e5..b9cab85 100644
--- a/libs/ui/GraphicBuffer.cpp
+++ b/libs/ui/GraphicBuffer.cpp
@@ -258,7 +258,12 @@
     mOwner = ownHandle;
 
     if (handle != 0) {
-        mBufferMapper.registerBuffer(handle);
+        status_t err = mBufferMapper.registerBuffer(handle);
+        if (err != NO_ERROR) {
+            ALOGE("unflatten: registerBuffer failed: %s (%d)",
+                    strerror(-err), err);
+            return err;
+        }
     }
 
     return NO_ERROR;