Enclose ExecContext._run() in an open() / close() - pair. This is
convenient for the caller in that it doesn't need to take care of
opening and closing for one call only, and inconvenient in that it
forces the caller to conciously add an open() / close() - pair around
multiple run() calls where it wants the context to stay open in
between. Or use the ExecContext as a context manager.
Signed-off-by: Jan Lindemann <jan@janware.com>