Improve error message when child dies

[Mic92]

Currently we use a very adhoc way of communicating with our eval process based on a line based protocol
We are using readLine which if the worker dies just return EndOfFile("unexpected EOF reading a line");.
We should replace this with just read followed by a wait on the child process to get its exit status.

[runeksvendsen]

Try evaluating default.nix in nixpkgs 20.09 to reproduce:

git clone -b 20.09 --single-branch https://github.com/NixOS/nixpkgs.git
nix-eval-jobs nixpkgs/default.nix 

output:

[...]

{"attr":"mandoc","attrPath":["mandoc"],"drvPath":"/nix/store/swvnn1pbp7ipg1rh6f39a9vgjpm7c0vm-mandoc-1.14.5.drv","name":"mandoc-1.14.5","outputs":{"out":"/nix/store/wfkxr2iljszzmffq4pddzwwjxzkw8zhq-mandoc-1.14.5"},"system":"x86_64-linux"}
error: unexpected EOF reading a line

Exit code is 1.

[runeksvendsen]

@Mic92 Actually, this looks like more than a poor error message. nix-eval-jobs seemingly dies trying to evaluate the "mandoc" attribute in nixpkgs 20.09.