recovery: add transparency to error images

Patch Set 1:    * added transparcency to following images

                    - icon_error.png
                    - icon_firmware_erroer.png
                    - icon_firmware_install.png

                * Preview:

                    old: http://bit.ly/ToTUUm
                    new: http://bit.ly/ToU3at

Change-Id: I1ff7957ecaeebd1f3dbabed7ab074ee709174f47