Change the meaning of the return value of SessionDirectory::create and add documentation to explain usage.
Add PBD::sys::filesystem_error to indicate a filesystem error and throw it where necessary.
Change the semantics of PBD::sys::create_directory/ies functions to match boost::filesystem
git-svn-id: svn://localhost/ardour2/trunk@1884 d708f5d6-7413-0410-9779-e7cbd77b26cf
b99c6c6e1d