elm-test --watch crashes if Elm code doesn't compile

[MartinSStewart]

Steps to reproduce:

1. Download sscce.zip and install NPM packages
2. Run yarn test in terminal
3. Notice that elm-test will start and then immediately crash with the following exception

(node:3644) UnhandledPromiseRejectionWarning: Compilation failed while attempting to run `elm make` on /Users/martinstewart/Desktop/sscce
(node:3644) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:3644) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Node version: v10.16.0
OS: MacOS
Elm version: 0.19.1

[lydell]

Update! It used to crash like this:

-- CYCLIC DEFINITION ------------------------------------------ src/MyModule.elm

The `a` value is defined directly in terms of itself, causing an infinite loop.

4| a = a
   ^
Are you are trying to mutate a variable? Elm does not have mutation, so when I
see a defined in terms of a, I treat it as a recursive definition. Try giving
the new value a new name!

Maybe you DO want a recursive value? To define a we need to know what a is, so
let’s expand it. Wait, but now we need to know what a is, so let’s expand it...
This will keep going infinitely!

Hint: The root problem is often a typo in some variable name, but I recommend
reading <https://elm-lang.org/0.19.1/bad-recursion> for more detailed advice,
especially if you actually do need a recursive value.

(node:93627) UnhandledPromiseRejectionWarning: Compilation failed while attempting to run `elm make` on /Users/lydell/Downloads/sscce
(Use `node --trace-warnings ...` to show where the warning was created)
(node:93627) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:93627) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Since #448 it exits like this:

-- CYCLIC DEFINITION ------------------------------------------ src/MyModule.elm

The `a` value is defined directly in terms of itself, causing an infinite loop.

4| a = a
   ^
Are you are trying to mutate a variable? Elm does not have mutation, so when I
see a defined in terms of a, I treat it as a recursive definition. Try giving
the new value a new name!

Maybe you DO want a recursive value? To define a we need to know what a is, so
let’s expand it. Wait, but now we need to know what a is, so let’s expand it...
This will keep going infinitely!

Hint: The root problem is often a typo in some variable name, but I recommend
reading <https://elm-lang.org/0.19.1/bad-recursion> for more detailed advice,
especially if you actually do need a recursive value.

undefined

So:

• No more unhandled promise rejection warnings.
• There’s a weird undefined at the end.
• It feels like the watcher should still keep going and automatically try to run the tests again when the user has fixed their code.