project membership editing breaks if non-existant members are still listed

[wlhunt]

Attempting to add a new project member causes a server error, if there exists a previously added project member that is no longer verifiable through LDAP. This situation occurs when a listed project member leaves the employment records.

Behavior should be modified to:
-catch the LDAP error
-warn the user that the current list has invalid members
-inform the user that invalid member will be removed
-allow the modified list (the new member and removed member) to be saved
-allow the user to cancel the operation