Update to latest Joi, allow validation errors to be handled by user

[parkan]

This introduces two major changes:

1. upgrade Joi to 7.x, which among other things throws Error with name: ValidationError, and update the callsites and tests accordingly
2. undo f940de6c4997ebacb95aebbbb80188fa72b69066, which made it impossible to handle validation errors in any custom way, and adds appropriate documentation and example in tests

[parkan]

the tests failure seems to be from mocha trying to recursively validate node_modules?