codewriter: return at first if CodeWriter.err is not nil
codewriter: return at first if CodeWriter.err is not nil

Signed-off-by: Soshi Katsuta <37be56e1b0b2d8665f687408a43d44ca133648fd@gmail.com>

skatsuta/nand2tetris,skatsuta/nand2tetris,skatsuta/nand2tetris