Rename menu background to menubg.png, move unknown_block.png and unknown_object.png to client, make generating a placeholder image for the filename "" not cause an error

src/main.cpp

@@ -1021,7 +1021,7 @@ void drawMenuBackground(video::IVideoDriver* driver)
 	core::dimension2d<u32> screensize = driver->getScreenSize();
 		
 	video::ITexture *bgtexture =
-			driver->getTexture(getTexturePath("mud.png").c_str());
+			driver->getTexture(getTexturePath("menubg.png").c_str());
 	if(bgtexture)
 	{
 		s32 texturesize = 128;

src/tile.cpp

@@ -1088,10 +1088,12 @@ bool generate_image(std::string part_of_name, video::IImage *& baseimg,
 
 		if(image == NULL)
 		{
+			if(part_of_name != ""){
 				errorstream<<"generate_image(): Could not load image \""
 						<<part_of_name<<"\""<<" while building texture"<<std::endl;
 				errorstream<<"generate_image(): Creating a dummy"
 						<<" image for \""<<part_of_name<<"\""<<std::endl;
+			}
 
 			// Just create a dummy image
 			//core::dimension2d<u32> dim(2,2);