Config: Validate all paths and throw an error if any are invalid

Ref https://github.com/mrjoelkemp/Dependents/issues/212

This would let a user know whether or not there is a mistake in their .deprc file.
- This should only print the error message, not stack trace.

No file or test is named in the issue. Begin at the configuration-loading entry point and trace how paths from .deprc are handled; reproduce an invalid path and inspect the existing error flow. Done means every configured path is validated and an invalid configuration prints only an error message, without a stack trace.

Written by the indexing model from the issue text.