Chain exports exits in success when daemon is killed

[travisperson]

Describe the bug
When running a chain export, if the daemon goes away (connection closes, dies, etc) the chain export will exit in success. This makes it really hard to ensure a proper export.

In this example, I issued a kill -15

➜  ~ lotus chain export --tipset @92000 test.car
➜  ~ echo $status
0

In this example, I issued a kill -9

➜  ~ lotus chain export --tipset @92000 test.car
2020-09-29T19:36:56.555Z	WARN	rpc	go-jsonrpc@v0.1.2-0.20200822201400-474f4fdccc52/websocket.go:660	failed to write close message: write tcp 127.0.0.1:35346->127.0.0.1:1234: write: broken pipe
➜  ~ echo $status
0