Fail if JS crypto is not available.

Emscripten registers /dev/urandom and /dev/random by default, but falls back to
Math.random()*256 when crypto.getRandomValues() is not available, which is not
acceptable. So we need to re-register them no matter what.

test/default/pre.js

@@ -23,7 +23,9 @@ Module['preRun'].push(function(){
             }
             randombyte_node();
             randombyte = randombyte_node;
-        } catch(e) { }
+        } catch(e) {
+            throw 'No secure random number generator found';
+        }
     }
     FS.init();
     var devFolder = FS.findObject('/dev') ||