Exit codes are not reported

Sessions opened with the session manager plugin always report an exit code of zero even when the last executed command within the session returns a non-zero value. For example:

```
$ aws ssm start-session --target [ec2 instance]
Starting session with SessionId: [name]-0ac90007f8af508b9
$ exit 1
Exiting session with sessionId: [name]-0ac90007f8af508b9.

$ echo $?
0
```

Compare this to SSH where the exit code is carried over:

```
$ ssh [host]
[host]:~$ exit 1
logout
Connection to [host] closed.

$ echo $?
1
```

Not reporting the exit code makes scripting extremely difficult since there's no way to tell if a given command was successful or not. Reporting the exit code of the last command run in the session would be more inline with other standard tools.