make sure missing map icons get a generic one instead of an error

[timwaters]

from #207 when theres over 150 or so points on a map, subsequent points have their icons missing. We should show a generic one.

[mrAceT]

What might be the perfect solution: Instead of an image do this: <div class="icon">123</div> and in the CSS some nice styling (linke centering the number) and use as a background (the marker image) a SVG file, that'would be perfect!

PS: I sent you a mail