CORE: Fixed type of returned exception for LoA

- When member is checked for possible extension and fails,
  we must return INSUFFICIENTLOAFOREXTENSION type of exception,
  so GUI will perform redirect.

  There originally was INSUFFICIENTLOA which is case only when user
  is not yet member of VO.

- Added javadoc for methods canBeMemberInternal(), manageMembershipExpiration().