When it's missing, we write a short message saying so, and when it's invalid, we do approximately the same thing. Athens has a lot of configuration options, so it's naturally easy to get them wrong or forget something. Nicer error messages should help.

If this code is acceptable, I'd like to apply it to a few other config values (in a follow-up PR) that are easy to get wrong.

ref gomods#1491, since this checks existence and validates values for download mode, and also reports errors in a more human-friendly way

cc/ @bluekeyes

cc/ @marwan-at-work, I think I've addressed almost all of your comments here, but let me know if this overall design is what you had in mind. Tests are still failing, I'm fixing those shortly

and setting a default

Also removing the op from DownloadFile.Validate

Just checking to see if there's an error. Also adding tests for valid modes

Looks like validate tags don't have a prefix checker. This seems easier than building a custom validator

…msgs