collect all errors before exiting validation

[matthewcornell]

per @bsweger comment:

The the change I do think we should make is bunching together error messages in the validations, so people don't have to make one change at a time to get this working (i.e., they get a list of everything that didn't validate so they can fix it all up)

And this one:

If there are multiple validation errors (e.g., missing quantile levels and not a designated model), it would be user-friendly to return all of them at once (instead of raising an error on the first failure).

Originally posted by @bsweger in #10 (review)