print warning on startup if path to configuration file does not exist

[lxxxvi]

Hi 👋

Here's my attempt to resolve #632

I suggest to display a WARNING in the terminal if the path to configuration file does not exist:

$ bin/jobs -c /path/to/nowhere.yml
[solid_queue] WARNING: Provided configuration file '/path/to/nowhere.yml' does not exist. Falling back to default configuration.

Let's discuss the approach and, of course, the message itself.

Looking forward to hearing your feedback.

[rosa]

Thanks a lot @lxxxvi! This looks good, just left a minor comment.