Improve error message when using wrong Crowdtilt API keys

[levin-ilya]

No description provided.

[liuhenry]

Hey Ilya, thanks for the PR. This should be helpful to those starting the app for the first time, and we could probably also do other improvements to the .env variables in the future, like trimming whitespace or warning of invalid characters.

We don't have any sort of style guide at the moment, but do you think you could change some of the spacing so that it matches our existing code? I've added the lines in the diff comments. Once that's done, looks like it'd be ready to merge!