JsChecker still warns (instead of erroring) for some classes of errors

```
INFO: From Checking 13 JS files in //workout:workout_lib:
workout/create.js:114: WARNING - Duplicate case in a switch statement.
case ResponseType.INVITE_USERS:
^

0 error(s), 1 warning(s)

ERROR: /home/ahochhaus/ll/workout/BUILD:69:1: Compiling 938 JavaScript files to workout/workout_bin.js failed: Worker process sent response with exit code: 1.
workout/create.js:114: ERROR - Duplicate case in a switch statement.
case ResponseType.INVITE_USERS:
^

1 error(s), 0 warning(s)
```

Using version

```
git_repository(
name = "io_bazel_rules_closure",
commit = "a072e282ee22935d1e43f78b60f4b702688fa801",
remote = "https://github.com/bazelbuild/rules_closure.git",
)
```

Reproduce the diagnostic using the pinned rules_closure commit and the example at workout/create.js:114, then inspect the related build invocation at BUILD:69. Confirm why the duplicate-case diagnostic is reported as a warning before compilation reports it as an error; done means the checker consistently reports this class of error with the expected severity.

Written by the indexing model from the issue text.