Bug 674430 - Improve error messages for attribute uniqueness
    
    The attribute uniqueness plug-in always returns the same error
    message when it encounters a problem.  The error message indicates
    that there is a uniqueness conflict even when that is not the case.
    
    This patch makes the error message only report a uniqueness conflict
    when there really is a uniqueness conflict.  All other errors will
    report that there was a problem checking for uniqueness.  This patch
    also makes a MODRN return an error 32 when the target does not exist
    instead of returning an operations error.