When loading sessions, create any missing session directories rather than throwing...

When loading sessions, create any missing session directories rather than throwing an exception.

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