fix: warning UnhandledPromiseRejectionWarning not throwing with node < 15

[MathieuPuech]

When you use rocket lint with a version prior to node 15, when you have links errors it does not throw an error but emit a warning:

(Use `node --trace-warnings ...` to show where the warning was created)
(node:49856) 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:49856) [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.

The exit code of the command is 0 and is not considered as an error.

Node has changed this behavior from the version 15.0.0:

v15.0.0 Changed default mode to throw. Previously, a warning was emitted.

See https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode

I didn't changed the behavior your have on node 15 but it could be good to catch these errors and do a process exit instead.

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Latest commit: 86ac08b6f7dcaf5e6d095c9d6f46bc62b7e6cd62

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