Try to handle no MediaSource case more gracefully.

Not sure how kosher it is to do things this way as far as w-p-t conventions
go, but this change:

  1. adds one additional simple test for typeof MediaSource != "undefined"
  before running the rest of the script

  2. adds a try-catch around the whole script for this test case, to cause
  any exceptions to just get logged to the console

That way, in UAs that doen't support MediaSource at all, instead of the
"Harness encountered an error" message and an exception message like "Can't
find variable: MediaSource" getting output to the page, just one clear test
failure gets displayed in the Summary and Details report part of the page.