Simplify visit parameters, handle all errors

[stephen-soltesz]

This change simplifies the parameters for the visit callback passed to WalkSchema and correctly handles an error case previously ignored.

---

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 593

• 13 of 15 (86.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.08%) to 82.359%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
bqx/schema.go	13	15	86.67%
<!--	Total:	13	15	86.67%	-->

Totals
Change from base Build 590:	-0.08%
Covered Lines:	845
Relevant Lines:	1026

---

💛 - Coveralls

[pboothe]

Please don't add uncovered lines. You don't need to clean up the campsite, but please don't leave it worse than you found it :)

[stephen-soltesz]

This did not go how I imagined :)

I'll address comments relevant to the original files in a separate PR.