Simplify validation, resolve gocognit warnings

Create internal validation helper and use it to replace existing
validation logic used in methods identified by gocognit. The end
result is that the larger, more complex validation methods are
(hopefully) more concise and readable now.

I began the process of replacing the logic used by other validation
methods with validation helper calls, but have not finished the
conversion work. I plan to return to this with a future PR once
I've reviewed the remaining (unconverted) validation logic further.

Credit to Fabrizio Milo for sharing the original implementation:

- https://stackoverflow.com/a/23960293/903870
- https://github.com/Mistobaan

refs GH-157