Set error on null etag
Set error on null etag

Original git commit by Alexander Larsson <alexl@redhat.com> at 1185358107 +0200

svn path=/trunk/; revision=707

gicmo/gvfs,xkahn/gvfs-cmis,philipl/gvfs,gicmo/gvfs,gicmo/gvfs,halfline/gvfs,halfline/gvfs,xkahn/gvfs-cmis,philipl/gvfs,thiblahute/gvfs,cpatulea/gvfs,halfline/gvfs,xkahn/gvfs-cmis,gicmo/gvfs,xkahn/gvfs-cmis,thiblahute/gvfs,cpatulea/gvfs,philipl/gvfs