Cleanup logs around io streams shutting down

Right now when a container exits you get some confusing error logs like this:
```
time="2019-12-11T20:03:38.436612731Z" level=error msg="error copying io" ExecID= TaskID=container-80-3 error="read unix @->firecracker.vsock: use of closed network connection" runtime=aws.firecracker stream=stdout vmID=80
```

or this:
```
time="2019-12-11T20:03:38.443037999Z" level=debug msg="[ 81.326403] agent[848]: time=\"2019-12-11T20:03:38Z\" level=error msg=\"error copying io\" ExecID= TaskID=container-80-3 error=\"read /proc/self/fd/76: file already closed\" stream=stderr" jailer=noop runtime=aws.firecracker vmID=80 vmm_stream=stdout
```

I don't think these in particular should be logged at ERROR level and they should probably be clarified as they just indicate the stream got closed. Something more like `std closed` at INFO level seems more appropriate. We can include the full actual error message at DEBUG still. We'll also want to make sure that if other unexpected errors occur, we continue to log those at ERROR.