[SPARK-11824][WEBUI] WebUI does not render descriptions with 'bad' HTML, throws console error

Don't warn when description isn't valid HTML since it may properly be like "SELECT ... where foo <= 1"

The tests for this code indicate that it's normal to handle strings like this that don't contain HTML as a string rather than markup. Hence logging every such instance as a warning is too noisy since it's not a problem. this is an issue for stages whose name contain SQL like the above

CC tdas as author of this bit of code

Author: Sean Owen <sowen@cloudera.com>

Closes #10159 from srowen/SPARK-11824.