On Error, switch to default framebuffer before showing bluescreen.

Bluescreens might not be shown if the error happens with a render target
different from the default framebuffer, because it gets rendered to
the attached framebuffer object. Fix that.

src/scripts/boot.lua

@@ -787,6 +787,8 @@ function love.errhand(msg)
 		return
 	end
 
+	love.graphics.setRenderTarget()
+
 	-- Load.
 	love.graphics.setScissor()
 	love.graphics.setBackgroundColor(89, 157, 220)