Bug 1611547: Problem with userlist forms

The forms are not rendering correctly now that we have removed
mochikit from mahara.js

The problem is the typeof(message) check can return 'object' when the
message is null - see http://www.ecma-international.org/ecma-262/5.1/#sec-11.4.3



So need to avoid trying to get the sub message from null object

Also added some accessibility strings to the Institutions -> Members
add form

Change-Id: Ic0a02ef35d5d0ed1844ab5792542fe0872e5376f
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>