[BUGFIX] Thumbnail placeholder: image-not-rendered is unreadable

If a thumbnail can not be created (im/gm mis-configuration or
inability to convert image format), a placeholder is rendered.
This placeholder contains text with further details.
This text is not readable, with white text on yellow background.
This patch changes: Black text on White background.

Change-Id: I3fa52b620a758caa0c864105203b314783319692
Resolves: #28962
Releases: 4.6
Reviewed-on: http://review.typo3.org/4308
Reviewed-by: Christopher 
Tested-by: Christopher 
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn