Use faultstring attribute in suds.WebFault.fault

WebFault handling fails with AttributeError if there is no element at
path '/Envelope/Body/Fault/faultstring'. This patch fixes the problem
by using faultstring attribute in WebFault.fault.

Closes-Bug: #1371547
Change-Id: Ic61b22f833dd82f4c4e2d424f320526d1bd8eacd
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