fix: Show error message if config file missing

[chardos]

Currently, if playroom.config.js is missing, it will throw an ambiguous error:

(node:21821) UnhandledPromiseRejectionWarning: AssertionError [ERR_ASSERTION]: missing path
    at Module.require (internal/modules/cjs/loader.js:596:3)
    at require (internal/modules/cjs/helpers.js:11:18)
    at /Users/richardnew/git/seek-ads/node_modules/playroom/bin/cli.js:58:18
    at <anonymous>
(node:21821) 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:21821) [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 README.md does cover what is needed, however since I bumped into this error myself, I thought a helpful hint may help the next person who neglects to RTFM.