Call RoClearError when handling device lost errors

This prevents the error state from staying around, which can cause
problems if there's a later error that doesn't call RoOriginateError.

XAML uses the error state to override the error code.  The result here
is that the UnhandledException handler will get called with the
device lost error state, rather than the real error state that was unhandled.

This is related to issue #194 where it looked like a device lost exception
was being unhandled, when instead it was another error.

winrt/lib/xaml/RecreatableDeviceManager.impl.h

@@ -407,6 +407,17 @@ namespace ABI { namespace Microsoft { namespace Graphics { namespace Canvas { na
             // This function must be called from inside a catch block.
             assert(std::current_exception());
 
+            //
+            // When the exception we're handling was thrown, we also called RoOriginateError.
+            //
+            // We need to clear this.  
+            //
+            // Otherwise, if any other errors occur that don't call RoOriginateError, 
+            // this stored error state will override any subsequent errors, resulting
+            // in confusing UnhandledException reporting.
+            //
+            RoClearError();
+
             // No m_device means that this exception has already been handled.
             if (!m_device)
                 return;