Fix UnboundLocalError during WebFault handling

If the document attribute of WebFault is unset, the 'detail' local
variable will be referenced before assignment which results in
UnboundLocalError. This patch fixes the problem by initializing
'detail' to None.

Change-Id: Icf17503f46eaa389b1f31dfffc84f21f69cde3aa
Closes-Bug: #1376172