No error when engine.t.output invalid

## env
```
(✓) 20:26:52 [~/code/milktest]
$ uname -a
Linux **** 5.0.0-31-generic #33~18.04.1-Ubuntu SMP Tue Oct 1 10:20:39 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
(✓) 20:27:00 [~/code/milktest]
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Pop!_OS 18.04 LTS
Release: 18.04
Codename: bionic
```
## Recreate
1. Make a "bad" path as types.json.engine.t.output (I typo'd `",/src/Serde.re"`), with a leading comma.
2. Run milk
3. milk exits with no error code or message

## What Happened
No Serde.re file created

## What I expected to happen
milk would exit with non-zero code or print an error as a result of invalid file system operation

Reproduce the issue using the invalid types.json.engine.t.output path and the leading-comma typo described in the report. Trace how milk handles the requested Serde.re output, then verify that the invalid filesystem operation produces an error or non-zero exit status and no misleading success.

Written by the indexing model from the issue text.