Remove dataset pkg

[stephen-soltesz]

This change removes the only remaining references to the go/dataset package by including error declarations in cloud/bq/ops.go instead.

---

This change is 

[stephen-soltesz]

Part of:

• https://github.com/m-lab/go/issues/88

[coveralls]

Pull Request Test Coverage Report for Build 3650

• 0 of 9 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.442%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cloud/bq/ops.go	0	9	0.0%
<!--	Total:	0	9	0.0%	-->

Totals
Change from base Build 3646:	0.0%
Covered Lines:	1259
Relevant Lines:	1605

---

💛 - Coveralls