This ensures that all el-get notifications will be logged in the
Messages buffer by default.
Note that the original default of "graphical" was chosen to be be
consistent with the behavior of el-get before the preference was
introduced.
Fixes#519
Notify functions can sometimes fail (due to dbus errors and such), and
this should not cause el-get to halt, because notification is arguably
non-essential. If the chosen notification method fails, "message" is
used instead.
