chore: move console messages to their own module

[jaebradley]

Summary

The console.error messages are effectively constants that make reading the logic in index.js slightly more tedious.

By moving these message strings to their own module, this slightly cleans up the logic in index.js.

Also, this adds some tests for whether these console.errors were called with the appropriate text.

Definitely understandable if this isn't merged as this type of code organization can be more a matter of taste than anything else.