DAVFS: use response text for error details, not response object itself.
DAVFS: use response text for error details, not response object itself.

This should make the errors easier to move around, e.g. pickling them and
sending them to a remote process. 


git-svn-id: 74b2def6592cf29d88d1a5d33b5c4a2732d8507c@505 67cdc799-7952-0410-af00-57a81ceafa0f

atty303/pyfilesystem,lightkeeper/lspyfilesystem,lightkeeper/lspyfilesystem