ENH: Add logging to extension update check

Add logging messages to report status when checking for updates, and
when an update is scheduled. (In particular, this reports a warning if
the server extension information query fails.)

Issue #3784

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23517 3bd1e089-480b-0410-8dfb-8563597acbee