sftp: return proper error on mkdir when directory exists
sftp: return proper error on mkdir when directory exists

Nautilus depends on properly reported errors when an operation fails.
In this case, user was not able to create new directory on sftp share
when "untitled folder" already existed.

SFTP only returns generic SSH_FX_FAILURE code so let's do SSH_FXP_LSTAT
on the target file and return proper error.

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