spi2davex: InvalidItemStateException not properly extracted from ambiguous response error

NodeTest#testSaveInvalidStateException
SessionTest#testSaveInvalidStateException

fail with PathNotFoundException instead of InvalidItemStateException.

i remember that i already addressed that issue in spi2dav a long time ago. with the batched writing in
spi2davex it is back: the server isn't aware of the distinction and just isn't able to retrieve that removed
item... either the client side finds a way to distinguish between path-not-found and externally modified
or we have to leave this as known issue...

in spi2dav i added add quick hack: if the operation was some write operation the path-not-found is
simply converted into invaliditemstateexception.