[simple] ImageLayout.watch: `img_load_error.svg` if img load fails

Show default image `img/img_load_error.svg` when image can't be loaded.

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Some words about class ImageLayout:

The https://github.com/searxng/searxng/blob/master/searx/static/themes/simple/js/searxng.js is build by a grunt task ..

https://github.com/searxng/searxng/blob/d0e21a01b4e14df905f25445a3f6bcbec481671c/searx/static/themes/simple/gruntfile.js#L91-L93

The `/__common__/js/*.js` concats also https://github.com/searxng/searxng/blob/master/searx/static/themes/__common__/js/image_layout.js where a modified copy of the of the "Google-image-layout" (`ImageLayout`) is implemented [1][2][3].

[1] https://ptgamr.github.io/2014-09-12-google-image-layout/
[2] https://ptgamr.github.io/google-image-layout/src/google-image-layout.js
[3] https://github.com/ptgamr/google-image-layout/tree/master


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Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>