Currency error instead of unit

[coveralls]

Pull Request Test Coverage Report for Build 1559265742

• 4 of 10 (40.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 36.383%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
primitives/src/currency.rs	4	10	40.0%
<!--	Total:	4	10	40.0%	-->

Totals
Change from base Build 1550209200:	0.1%
Covered Lines:	118
Relevant Lines:	277

---

💛 - Coveralls