hosted_engine_page: replace RuntimeError with InfoDialog

Currently if the plugin cannot download ISO/OVA image it
throws a RuntimeError to users. This patch replace the
RuntimeError with userfriendly InfoDialog call.

Change-Id: I7f2aadc7e743da030e6d61a01695af33faff1cfc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1127683
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>