fix #974 test.aria.utils.cfgframe.AriaWindowTest failing on IE 8

Commit 2c1141e33deb90e8a5c5cd10676d87b486dd22c4 makes the following
test fail on IE 8: test.aria.utils.cfgframe.AriaWindowTest.
It only happens on a true IE 8 (not on IE 11 in IE 8 mode).

The issue comes from an "Unspecified error" raised when trying to read
Aria.$window.document.activeElement. This commit surrounds that piece
of code by a try ... catch.