Card - improve error message when reading invalid svg files

Add a test file in samples/images - it's a simple 1x1 pixel white png
saved with a 'svg' extension.

Fix for #228

(cherry picked from commit 32fad2460753a72cc2b0100d0a132512b6eeed81)

# Conflicts:
#	lib/squib/graphics/image.rb

samples/images/_more_load_images.rb

@@ -95,5 +95,8 @@ Squib::Deck.new(width: 825, height: 1125, cards: 1, config: 'load_images_config.
   svg file: 'spanner.svg', width: :deck, height: :deck,
       force_id: true, id: '' # <-- the important part
 
+  # Squib can handle incorrectly formatted svg files
+  svg file: 'bad-svg.svg'
+
   save prefix: 'load_images_', format: :png
 end