start: Fix errors propagation

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
headers: Fix path to errors.h

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error for invalid argument

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Error for unsupported operations

E.g. -- sb spawn and enter for unix session.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
err: Add detailed errors about cgroups

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>