Load a placeholder when a url to an image is broken.

I decided to use the old Netscape broken image link icon (later we may
replace the image asset for something more trendier). The ref test will
expect that a failed load should display the rippy image.

ImageCacheTask users can define if a placeholder image should be loaded
at start up or not. This enables both the new behavior (e.g. always
return an image even for broken urls) as also the previous one.

tests/ref/basic.list

@@ -193,6 +193,7 @@ flaky_cpu == linebreak_simple_a.html linebreak_simple_b.html
 == multiple_css_class_a.html multiple_css_class_b.html
 == negative_margin_uncle_a.html negative_margin_uncle_b.html
 == negative_margins_a.html negative_margins_b.html
+== no-image.html no-image-ref.html
 == noscript.html noscript_ref.html
 != noteq_attr_exists_selector.html attr_exists_selector_ref.html
 == nth_child_pseudo_a.html nth_child_pseudo_b.html

tests/ref/no-image-ref.html

@@ -0,0 +1 @@
+<img width="100px" height="100px" src="rippy.jpg">

tests/ref/no-image.html

@@ -0,0 +1 @@
+<img width="100px" height="100px" src="i-feel-a-disturbance-in-the-force.wtf">